[release-branch.go1.17] syscall: 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.

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#52313
Fixes golang#52439

Change-Id: I4e2c84754b0af7830b40fd15dedcbc58374d75ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/399539
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit f66925e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/401078
Auto-Submit: Tatiana Bradley <tatiana@golang.org>
Run-TryBot: Tatiana Bradley <tatiana@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Tatiana Bradley <tatiana@golang.org>

src/syscall/syscall_linux.go

@@ -106,7 +106,7 @@ 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