nilfs2: fix lockdep recursive locking warning on bmap

The bmap semaphore of DAT file can be held while a bmap of other files
is locked.  This has caused the following false detection of lockdep
check:

mount.nilfs2/4667 is trying to acquire lock:
 (&bmap->b_sem){..--}, at: [<d0c6c4b4>] nilfs_bmap_lookup_at_level+0x1a/0x74 [nilfs2]

but task is already holding lock:
 (&bmap->b_sem){..--}, at: [<d0c6c4b4>] nilfs_bmap_lookup_at_level+0x1a/0x74 [nilfs2]

This will fix the false detection by distinguishing semaphores of the
DAT and other files.

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