umtx: handle allocation failire in umtx_pi_alloc()

Don't assume that this allocation will succeed. We may have been passed
M_NOWAIT.

The calling code already handles allocation failures, but the function
itself did not.

PR: 283807
MFC after: 1 week

(cherry picked from commit 50c1e179b584f43ba82e9afc91b25ec4831b58ef)