nfsd4: simplify lock openmode check

Note that the special handling for the lock stateid case is already done
by nfs4_check_openmode() (as of 02921914170e3b7fea1cd82dac9713685d2de5e2
"nfsd4: fix openmode checking on IO using lock stateid") so we no longer
need these two cases in the caller.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>