Let linuxulator mprotect mask unsupported bits before calling kern_mprotect.

After r349240 kern_mprotect returns EINVAL for unsupported bits in the prot
argument. Linux rtld uses PROT_GROWSDOWN and PROT_GROWS_UP when marking the
stack executable. Mask these bits like kern_mprotect used to do. For other
unsupported bits EINVAL is returned like Linux does.

Reviewed by: trasz, brooks
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20864