vfs: remove production kernel checks and mp == NULL support from vdrop

1. The only place in the tree which calls getnewvnode with mp == NULL does it

for vp_crossmp which will never execute this codepath. Any vnode which legally
has ->v_mount == NULL is also doomed, which once more wont execute this code.

2. Remove an assertion for v_holdcnt from production kernels. It gets taken care

of by refcount macros in debug kernels.

Any code which would want to pass NULL mp can construct a fake one instead.

Reviewed by: kib (previous version)
Differential Revision: https://reviews.freebsd.org/D22722