nilfs2: always lookup disk block address before reading metadata block

The current metadata file code skips disk address lookup for its data
block if the buffer has a mapped flag.

This has a potential risk to cause read request to be performed
against the stale block address that GC moved, and it may lead to meta
data corruption.  The mapped flag is safe if the buffer has an
uptodate flag, otherwise it may prevent necessary update of disk
address in the next read.

This will avoid the potential problem by ensuring disk address lookup
before reading metadata block even for buffers with the mapped flag.

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