Fix Linux recvmsg(2) when msg_namelen returned is 0. Previously
it would fail with EINVAL, breaking some of the Python regression
tests.

While here, cap the user-controlled message length.

Note that the code doesn't seem to be copying out the new length
in either (success or failure) case. This will be addressed separately.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25392