NFSD: Disallow layoutget during grace period

When the server is recovering from a reboot and is in a grace period,
any operation that may result in deletion or reallocation of block
extents should not be allowed. See RFC 8881, section 18.43.3.

If multiple clients write data to the same file, rebooting the server
during writing may result in file corruption. In the worst case, the
exported XFS may also become corrupted. Observed this behavior while
testing pNFS block volume setup.

Co-developed-by: Konstantin Evtushenko <koevtushenko@yandex.com>
Signed-off-by: Konstantin Evtushenko <koevtushenko@yandex.com>
Signed-off-by: Sergey Bashirov <sergeybashirov@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>