nfsd: Remove unnecessary assignment in nfs4xdr.c

In nfsd4_encode_listxattrs(), the variable p is assigned to at one point
but this value is never used before p is reassigned. Fix this.

Addresses-Coverity: ("Unused value")
Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>