NFSv4/flexfiles: fix to allocate mirror->dss before use

Move mirror_array's dss_count initialization and dss allocation to
ff_layout_alloc_mirror(), just before the loop that initializes each
nfs4_ff_layout_ds_stripe's nfs_file_localio.

Also handle NULL return from kcalloc() and remove one level of indent
in ff_layout_alloc_mirror().

This commit fixes dangling nfsd_serv refcount issues seen when using
NFS LOCALIO and then attempting to stop the NFSD service.

Fixes: 20b1d75f ("NFSv4/flexfiles: Add support for striped layouts")
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>