p9fs: use M_WAITOK where appropriate

device_attach routines are allowed to sleep, and this routine already
has other M_WAITOK allocations.

Reported by: markj
Reviewed by: markj
Fixes: 1efd69f933b6 ("p9fs: move NULL check immediately after alloc...")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45721