nfsd4: remove redundant "if" in nfs4_preprocess_stateid_op

Note that we exit this first big "if" with stp == NULL if and only if we
took the first branch; therefore, the second "if" is redundant, and we
can just combine the two, simplifying the logic.

Reviewed-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>