ext4: recheck buffer uptodate bit under buffer lock

Commit 8e33fadf ("ext4: remove an unnecessary if statement in
__ext4_get_inode_loc()") forget to recheck buffer's uptodate bit again
under buffer lock, which may overwrite the buffer if someone else have
already brought it uptodate and changed it.

Fixes: 8e33fadf ("ext4: remove an unnecessary if statement in __ext4_get_inode_loc()")
Cc: stable@kernel.org
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20210910080316.70421-1-yi.zhang@huawei.com