Fix a bug introduced in r283433.

[1] Remove unneeded sockaddr conversion before kern_recvit() call as the from
argument is used to record result (the source address of the received message) only.

[2] In Linux the type of msg_namelen member of struct msghdr is signed but native
msg_namelen has a unsigned type (socklen_t). So use the proper storage to fetch fromlen
from userspace and than check the user supplied value and return EINVAL if it is less
than 0 as a Linux do.

Reported by: Thomas Mueller <tmueller at sysgo dot com> [1]
Reviewed by: kib@
Approved by: re (gjb, kib)
MFC after: 3 days