NFS: fsync() should report filesystem errors over EINTR/ERESTARTSYS

If the commit to disk is interrupted, we should still first check for
filesystem errors so that we can report them in preference to the error
due to the signal.

Fixes: 2197e9b0 ("NFS: Fix up fsync() when the server rebooted")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>