nfsd: fix dead ACL conflict guard in nfsd4_create

In the Linux kernel, the following vulnerability has been resolved:

nfsd: fix dead ACL conflict guard in nfsd4_create

nfsd4_create() steals create->cr_dpacl/cr_pacl into the local
nfsd_attrs via the designated initializer, then immediately sets the
source pointers to NULL. The subsequent conflict guard tests the
already-nilled source fields, making it permanently dead code:

if (create->cr_acl) {
if (create->cr_dpacl || create->cr_pacl) /* always false */

When a client encodes both FATTR4_WORD0_ACL and
FATTR4_WORD2_POSIX_{DEFAULT,ACCESS}_ACL in the same CREATE fattr
bitmap, nfsd4_acl_to_attr() overwrites attrs.na_pacl/na_dpacl without
releasing the originals, leaking two posix_acl slab objects per
request. Repeated requests cause unbounded slab exhaustion.

Fix by checking attrs.na_dpacl/na_pacl (the stolen values) instead of
the nilled create->cr_dpacl/cr_pacl, matching the correct pattern
already used in nfsd4_setattr().