nilfs2: delete unnecessary condition in nilfs_dat_translate

This is a trivial patch to delete unnecessary condition in nilfs_dat_translate.

nilfs_dat_translate() will asign translated address to *blocknrp if blocknrp
is not NULL.  However the condition is unneeded, because all callers of
nilfs_dat_translate() pass blocknrp properly.

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