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

sys/compat/linux/linux_mmap.c

@@ -228,6 +228,11 @@ int
 linux_mprotect_common(struct thread *td, uintptr_t addr, size_t len, int prot)
 {
 
+	/* XXX Ignore PROT_GROWSDOWN and PROT_GROWSUP for now. */
+	prot &= ~(LINUX_PROT_GROWSDOWN | LINUX_PROT_GROWSUP);
+	if ((prot & ~(PROT_READ | PROT_WRITE | PROT_EXEC)) != 0)
+		return (EINVAL);
+
 #if defined(__amd64__)
 	linux_fixup_prot(td, &prot);
 #endif

sys/compat/linux/linux_mmap.h

@@ -41,6 +41,8 @@
 #define	LINUX_MAP_ANON		0x0020
 #define	LINUX_MAP_GROWSDOWN	0x0100
 
+#define	LINUX_PROT_GROWSDOWN	0x01000000
+#define	LINUX_PROT_GROWSUP	0x02000000
 
 int linux_mmap_common(struct thread *, uintptr_t, size_t, int, int,
 			int, off_t);