ext4: silence sparse check warning for function ext4_trim_extent

This fixes the following sparse warning:

     CHECK   fs/ext4/mballoc.c
   fs/ext4/mballoc.c:5019:9: warning: context imbalance in
   'ext4_trim_extent' - unexpected unlock

Signed-off-by: "Jon Ernst" <jonernst07@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -5016,6 +5016,8 @@ error_return:
  */
 static int ext4_trim_extent(struct super_block *sb, int start, int count,
 			     ext4_group_t group, struct ext4_buddy *e4b)
+__releases(bitlock)
+__acquires(bitlock)
 {
 	struct ext4_free_extent ex;
 	int ret = 0;