nfsd: use vfs_fsync_range(), not O_SYNC, for stable writes

NFSv4 shares the same struct file across multiple writes.  (And we'd
like NFSv2 and NFSv3 to do that as well some day.)

So setting O_SYNC on the struct file as a way to request a synchronous
write doesn't work.

Instead, do a vfs_fsync_range() in that case.

Reported-by: Peter Staubach <pstaubach@exagrid.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>