pnfs/flexfiles: validate ds_versions_cnt is non-zero

nfs4_ff_alloc_deviceid_node() reads version_count from XDR without
checking it is non-zero. When a malicious NFS server sends a pNFS
LAYOUTGET response with version_count=0, kcalloc(0, ...) returns
ZERO_SIZE_PTR (0x10). The subsequent ds_versions[0] access in
nfs4_ff_layout_ds_version() and other callers dereferences this
invalid pointer, causing an out-of-bounds read.

Add a check for version_count == 0 after parsing it from XDR, before
the allocation.

The OOB read was confirmed with KASAN: null-ptr-deref in range
[0x0000000000000010-0x0000000000000017] from accessing ZERO_SIZE_PTR.

Fixes: d67ae825a59d ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Reported-by: Klaudia Kloc <klaudia@vidocsecurity.com>
Reported-by: Dawid Moczadło <dawid@vidocsecurity.com>
Tested-by: Jenny Guanni Qu <qguanni@gmail.com>
Signed-off-by: Jenny Guanni Qu <qguanni@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>