btrfs: remove pointless readahead callback wrapper

There's no point in having a static readahead callback in inode.c that
does nothing besides calling extent_readahead() from extent_io.c.
So just remove the callback at inode.c and rename extent_readahead()
to btrfs_readahead().
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -2267,7 +2267,7 @@ int extent_writepages(struct address_space *mapping,
 	return ret;
 }
 
-void extent_readahead(struct readahead_control *rac)
+void btrfs_readahead(struct readahead_control *rac)
 {
 	struct btrfs_bio_ctrl bio_ctrl = { .opf = REQ_OP_READ | REQ_RAHEAD };
 	struct page *pagepool[16];

fs/btrfs/extent_io.h

@@ -241,7 +241,7 @@ int extent_writepages(struct address_space *mapping,
 		      struct writeback_control *wbc);
 int btree_write_cache_pages(struct address_space *mapping,
 			    struct writeback_control *wbc);
-void extent_readahead(struct readahead_control *rac);
+void btrfs_readahead(struct readahead_control *rac);
 int extent_fiemap(struct btrfs_inode *inode, struct fiemap_extent_info *fieinfo,
 		  u64 start, u64 len);
 int set_folio_extent_mapped(struct folio *folio);

fs/btrfs/inode.c

@@ -7929,11 +7929,6 @@ static int btrfs_writepages(struct address_space *mapping,
 	return extent_writepages(mapping, wbc);
 }
 
-static void btrfs_readahead(struct readahead_control *rac)
-{
-	extent_readahead(rac);
-}
-
 /*
  * For release_folio() and invalidate_folio() we have a race window where
  * folio_end_writeback() is called but the subpage spinlock is not yet released.