It will be used for the upcoming LRO hash table initialization.
And probably will be useful in other cases, when M_WAITOK can't
be used.
Reviewed by: jhb, kib
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D6138
Approved by: gnn
Add a new function hashinit_flags() which allows NOT-waiting
for memory (or waiting). The old hashinit() function now
calls hashinit_flags(..., HASH_WAITOK);
