NFSD: Clean up nfsd_vfs_write()

[ Upstream commit 33388b3a ]

The RWF_SYNC and !RWF_SYNC arms are now exactly alike except that
the RWF_SYNC arm resets the boot verifier twice in a row. Fix that
redundancy and de-duplicate the code.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>