kernel/vm: Prohibit offset from being < 0 in mmap.

POSIX seems to be silent about whether this is permitted,
but at least FreeBSD explicitly does not allow it.

Fixes #16936.

Change-Id: If6b23410ae88245e706df77d276234f4775654f9

src/system/kernel/vm/vm.cpp

@@ -941,6 +941,8 @@ map_backing_store(VMAddressSpace* addressSpace, VMCache* cache, off_t offset,
 #endif
 		return B_BAD_VALUE;
 	}
+	if (offset < 0)
+		return B_BAD_VALUE;
 
 	uint32 allocationFlags = HEAP_DONT_WAIT_FOR_MEMORY
 		| HEAP_DONT_LOCK_KERNEL_SPACE;