nfsd: nfsd_setattr needs to call commit_metadata

The conversion of write_inode_now calls to commit_metadata in commit
f501912a missed out the call in nfsd_setattr.

But without this conversion we can't guarantee that a SETATTR request
has actually been commited to disk with XFS, which causes a regression
from 2.6.32 (only for NFSv2, but anyway).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>