NFSD: Store parent's stat in a separate value

After commit ae7095a7 (nfsd4: helper function for getting mounted_on
ino) we ignore the return value from get_parent_attributes().

Also, the following FATTR4_WORD2_LAYOUT_BLKSIZE uses stat.blksize, so to
avoid overwriting that, use an independent value for the parent's
attributes.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>