btrfs: constify parameters of write_eb_member() and its users

Reported by 'gcc -Wcast-qual', the argument from which write_extent_buffer()
reads data to write to the eb should be const. In addition the const
needs to be also added to __write_extent_buffer() local buffers.

All callers of write_eb_member() can now be updated to use const for the
input buffer structure or type.
Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/accessors.h

@@ -48,8 +48,8 @@ static inline void put_unaligned_le8(u8 val, void *p)
 			    offsetof(type, member),			\
 			    sizeof_field(type, member)))
 
-#define write_eb_member(eb, ptr, type, member, result) (\
-	write_extent_buffer(eb, (char *)(result),			\
+#define write_eb_member(eb, ptr, type, member, source) (		\
+	write_extent_buffer(eb, (const char *)(source),			\
 			   ((unsigned long)(ptr)) +			\
 			    offsetof(type, member),			\
 			    sizeof_field(type, member)))
@@ -353,7 +353,7 @@ static inline void btrfs_tree_block_key(const struct extent_buffer *eb,
 
 static inline void btrfs_set_tree_block_key(const struct extent_buffer *eb,
 					    struct btrfs_tree_block_info *item,
-					    struct btrfs_disk_key *key)
+					    const struct btrfs_disk_key *key)
 {
 	write_eb_member(eb, item, struct btrfs_tree_block_info, key, key);
 }
@@ -446,7 +446,7 @@ void btrfs_node_key(const struct extent_buffer *eb,
 		    struct btrfs_disk_key *disk_key, int nr);
 
 static inline void btrfs_set_node_key(const struct extent_buffer *eb,
-				      struct btrfs_disk_key *disk_key, int nr)
+				      const struct btrfs_disk_key *disk_key, int nr)
 {
 	unsigned long ptr;
 
@@ -512,7 +512,7 @@ static inline void btrfs_item_key(const struct extent_buffer *eb,
 }
 
 static inline void btrfs_set_item_key(struct extent_buffer *eb,
-				      struct btrfs_disk_key *disk_key, int nr)
+				      const struct btrfs_disk_key *disk_key, int nr)
 {
 	struct btrfs_item *item = btrfs_item_nr(eb, nr);
 

fs/btrfs/extent_io.c

@@ -4606,7 +4606,7 @@ static void __write_extent_buffer(const struct extent_buffer *eb,
 	size_t cur;
 	size_t offset;
 	char *kaddr;
-	char *src = (char *)srcv;
+	const char *src = (const char *)srcv;
 	unsigned long i = get_eb_folio_index(eb, start);
 	/* For unmapped (dummy) ebs, no need to check their uptodate status. */
 	const bool check_uptodate = !test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags);