ext4: make __ext4_get_inode_loc plug

Add a blk_plug to prevent the inode table readahead from being
submitted as small I/O requests.

Signed-off-by: zhangjs <zachary@baishancloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>