Remove access() call from Snapsafe detection (#2197)

### Description of changes: 
This change uses stat() instead of access() to check for the existence
of the Sysgenid device. This is more accurate to ascertain whether the
device exists on a machine. access() could fail for various other
reasons such as missing permissions.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

crypto/fipsmodule/rand/snapsafe_detect.c

@@ -9,6 +9,7 @@
 #include <fcntl.h>
 #include <stdlib.h>
 #include <sys/mman.h>
+#include <sys/stat.h>
 #include <unistd.h>
 #include "../delocate.h"
 
@@ -21,28 +22,18 @@ DEFINE_STATIC_ONCE(aws_snapsafe_init)
 DEFINE_BSS_GET(volatile uint32_t *, sgc_addr)
 DEFINE_BSS_GET(int, snapsafety_state)
 
-// aws_snapsafe_check_kernel_support returns 1 if the special sysgenid device
-// file exists and 0 otherwise.
-static int aws_snapsafe_check_kernel_support(void) {
-  // This file-exist method is generally brittle. But for our purpose, this
-  // should be more than fine.
-  if (access(CRYPTO_get_sysgenid_path(), F_OK) != 0) {
-    return 0;
-  }
-  return 1;
-}
-
 static void do_aws_snapsafe_init(void) {
-  *snapsafety_state_bss_get() = SNAPSAFETY_STATE_NOT_SUPPORTED;
   *sgc_addr_bss_get() = NULL;
+  *snapsafety_state_bss_get() = SNAPSAFETY_STATE_NOT_SUPPORTED;
 
-  if (aws_snapsafe_check_kernel_support() != 1) {
+  struct stat buff;
+  if (stat(CRYPTO_get_sysgenid_path(), &buff) != 0) {
     return;
   }
-  *snapsafety_state_bss_get() = SNAPSAFETY_STATE_FAILED_INITIALISE;
 
+  *snapsafety_state_bss_get() = SNAPSAFETY_STATE_FAILED_INITIALISE;
   int fd_sgc = open(CRYPTO_get_sysgenid_path(), O_RDONLY);
-  if (fd_sgc == -1) {
+  if (fd_sgc < 0) {
     return;
   }