NFSD: pass range end to vfs_fsync_range() instead of count

_nfsd_copy_file_range() calls vfs_fsync_range() with an offset and
count (bytes written), but the former wants the start and end bytes
of the range to sync. Fix it up.

Fixes: eac0b17a ("NFSD add vfs_fsync after async copy is done")
Signed-off-by: Brian Foster <bfoster@redhat.com>
Tested-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>