unix: check correct group in Faccessat

The Faccessat call checks the user, group, or other permission bits of a
file to see if the calling process can access it. The test to see if the
group permissions should be used was made with the wrong group id, using
the process's group id rather than the file's group id. Fix this to use
the correct group id.

This change only affects Linux versions prior to 5.8. Linux 5.8 added
the faccessat2 system call, which we use in preference to the internal
implementation.

No test since we cannot easily change file permissions when not running
as root and the test is meaningless if running as root.

For golang/go#52313

Change-Id: I6fa64379a50c9380207eab9d095ef7fbd05a2d59
Reviewed-on: https://go-review.googlesource.com/c/sys/+/400074
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

unix/syscall_linux.go

@@ -2189,7 +2189,7 @@ func Faccessat(dirfd int, path string, mode uint32, flags int) (err error) {
 			gid = Getgid()
 		}
 
-		if uint32(gid) == st.Gid || isGroupMember(gid) {
+		if uint32(gid) == st.Gid || isGroupMember(int(st.Gid)) {
 			fmode = (st.Mode >> 3) & 7
 		} else {
 			fmode = st.Mode & 7