NFSD: Refactor nfsd_setattr()'s ACL error reporting

NFSD: Refactor nfsd_setattr()'s ACL error reporting

Support for FATTR4_POSIX_ACCESS_ACL and FATTR4_POSIX_DEFAULT_ACL
attributes in subsequent patches allows clients to set both ACL
types simultaneously during SETATTR and file creation. Each ACL
type can succeed or fail independently, requiring the server to
clear individual attribute bits in the reply bitmap when one
fails while the other succeeds.

The existing na_aclerr field cannot distinguish which ACL type
encountered an error. Separate error fields (na_paclerr for
access ACLs, na_dpaclerr for default ACLs) enable the server to
report per-ACL-type failures accurately.

This refactoring also adds validation previously absent: default
ACL processing rejects non-directory targets with EINVAL and
passes NULL to set_posix_acl() when a_count is zero to delete
the ACL. Access ACL processing rejects zero a_count with EINVAL
for ACL_SCOPE_FILE_SYSTEM semantics (the only scope currently
supported).

The changes preserve compatibility with existing NFSv4 ACL code.
NFSv4 ACL conversion (nfs4_acl_nfsv4_to_posix()) never produces
POSIX ACLs with a_count == 0, so the new validation logic only
affects future POSIX ACL attribute handling.

Signed-off-by: Rick Macklem <rmacklem@uoguelph.ca>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>