ext4: clean up s_mb_rb_lock to fix build warnings with C=1

ext4: clean up s_mb_rb_lock to fix build warnings with C=1

Running sparse (make C=1) on mballoc.c we get the following warning:

 fs/ext4/mballoc.c:3194:13: warning: context imbalance in
  'ext4_mb_seq_structs_summary_start' - wrong count at exit

This is because __acquires(&EXT4_SB(sb)->s_mb_rb_lock) was called in
ext4_mb_seq_structs_summary_start(), but s_mb_rb_lock was removed in commit
83e80a6e3543 ("ext4: use buckets for cr 1 block scan instead of rbtree"),
so remove the __acquires to silence the warning.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20240319113325.3110393-10-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>