nfsd: fix GET_DIR_DELEGATION when VFS leases are disabled

nfsd: fix GET_DIR_DELEGATION when VFS leases are disabled

When leases are disabled on the server, running xfstest generic/309 leads
to an error because GET_DIR_DELEGATION returns EINVAL.

nfsd_get_dir_deleg() can fail in several ways: like memory allocation and
unable to get a lease because either leases are disable or it's already
held. Currently only the condition "already held" is translated to
returning directory-delegation-is-unavailable error. However, other failure
conditions are likely temporary and thus should result in the same kind
of error.

Fixes: 8b99f6a8c116 ("nfsd: wire up GET_DIR_DELEGATION handling")
Cc: stable@vger.kernel.org
Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>