nilfs2: remove buffer locking in nilfs_mark_inode_dirty

This lock is eliminable because inodes on the buffer can be updated
independently.  Although a log writer also fills in bmap data on the
on-disk inodes, this update is exclusively done by a log writer lock.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>