btrfs: remove no longer used btrfs_next_extent_map()

There are no more users of btrfs_next_extent_map(), the previous patch
in the series ("btrfs: search for delalloc more efficiently during
lseek/fiemap") removed the last usage of the function, so delete it.

This change is part of a patchset that has the goal to make performance
better for applications that use lseek's SEEK_HOLE and SEEK_DATA modes to
iterate over the extents of a file. Two examples are the cp program from
coreutils 9.0+ and the tar program (when using its --sparse / -S option).
A sample test and results are listed in the changelog of the last patch
in the series:

  1/9 btrfs: remove leftover setting of EXTENT_UPTODATE state in an inode's io_tree
  2/9 btrfs: add an early exit when searching for delalloc range for lseek/fiemap
  3/9 btrfs: skip unnecessary delalloc searches during lseek/fiemap
  4/9 btrfs: search for delalloc more efficiently during lseek/fiemap
  5/9 btrfs: remove no longer used btrfs_next_extent_map()
  6/9 btrfs: allow passing a cached state record to count_range_bits()
  7/9 btrfs: update stale comment for count_range_bits()
  8/9 btrfs: use cached state when looking for delalloc ranges with fiemap
  9/9 btrfs: use cached state when looking for delalloc ranges with lseek
Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Link: https://lore.kernel.org/linux-btrfs/20221106073028.71F9.409509F4@e16-tech.com/
Link: https://lore.kernel.org/linux-btrfs/CAL3q7H5NSVicm7nYBJ7x8fFkDpno8z3PYt5aPU43Bajc1H0h1Q@mail.gmail.com/Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -529,35 +529,6 @@ static struct extent_map *next_extent_map(const struct extent_map *em)
 	return container_of(next, struct extent_map, rb_node);
 }
 
-/*
- * Get the extent map that immediately follows another one.
- *
- * @tree:       The extent map tree that the extent map belong to.
- *              Holding read or write access on the tree's lock is required.
- * @em:         An extent map from the given tree. The caller must ensure that
- *              between getting @em and between calling this function, the
- *              extent map @em is not removed from the tree - for example, by
- *              holding the tree's lock for the duration of those 2 operations.
- *
- * Returns the extent map that immediately follows @em, or NULL if @em is the
- * last extent map in the tree.
- */
-struct extent_map *btrfs_next_extent_map(const struct extent_map_tree *tree,
-					 const struct extent_map *em)
-{
-	struct extent_map *next;
-
-	/* The lock must be acquired either in read mode or write mode. */
-	lockdep_assert_held(&tree->lock);
-	ASSERT(extent_map_in_tree(em));
-
-	next = next_extent_map(em);
-	if (next)
-		refcount_inc(&next->refs);
-
-	return next;
-}
-
 static struct extent_map *prev_extent_map(struct extent_map *em)
 {
 	struct rb_node *prev;

fs/btrfs/extent_map.h

@@ -87,8 +87,6 @@ static inline u64 extent_map_block_end(struct extent_map *em)
 void extent_map_tree_init(struct extent_map_tree *tree);
 struct extent_map *lookup_extent_mapping(struct extent_map_tree *tree,
 					 u64 start, u64 len);
-struct extent_map *btrfs_next_extent_map(const struct extent_map_tree *tree,
-					 const struct extent_map *em);
 int add_extent_mapping(struct extent_map_tree *tree,
 		       struct extent_map *em, int modified);
 void remove_extent_mapping(struct extent_map_tree *tree, struct extent_map *em);