drop direct use of stat syscalls in fchmodat

instead, use the fstatat/stat functions, so that the logic for which
syscalls are present and usable is all in fstatat.

this results in a slight increase in cost for old kernels on 32-bit
archs: now statx will be attempted first rather than just using the
legacy time32 syscalls, despite us not caring about timestamps.
however, it's not even clear that the legacy syscalls *should* succeed
if the timestamps are out of range; arguably they should fail with
EOVERFLOW. as such, paying a small cost here on old kernels seems
well-motivated.

with this change, fchmodat itself is no longer blocking ports to new
archs that lack the legacy syscalls.

src/stat/fchmodat.c

@@ -2,7 +2,6 @@
 #include <fcntl.h>
 #include <errno.h>
 #include "syscall.h"
-#include "kstat.h"
 
 int fchmodat(int fd, const char *path, mode_t mode, int flag)
 {
@@ -11,12 +10,12 @@ int fchmodat(int fd, const char *path, mode_t mode, int flag)
 	if (flag != AT_SYMLINK_NOFOLLOW)
 		return __syscall_ret(-EINVAL);
 
-	struct kstat st;
+	struct stat st;
 	int ret, fd2;
 	char proc[15+3*sizeof(int)];
 
-	if ((ret = __syscall(SYS_fstatat, fd, path, &st, flag)))
+	if (fstatat(fd, path, &st, flag))
-		return __syscall_ret(ret);
+		return -1;
 	if (S_ISLNK(st.st_mode))
 		return __syscall_ret(-EOPNOTSUPP);
 
@@ -27,12 +26,12 @@ int fchmodat(int fd, const char *path, mode_t mode, int flag)
 	}
 
 	__procfdname(proc, fd2);
-	ret = __syscall(SYS_fstatat, AT_FDCWD, proc, &st, 0);
+	ret = stat(proc, &st);
 	if (!ret) {
-		if (S_ISLNK(st.st_mode)) ret = -EOPNOTSUPP;
+		if (S_ISLNK(st.st_mode)) ret = __syscall_ret(-EOPNOTSUPP);
-		else ret = __syscall(SYS_fchmodat, AT_FDCWD, proc, mode);
+		else ret = syscall(SYS_fchmodat, AT_FDCWD, proc, mode);
 	}
 
 	__syscall(SYS_close, fd2);
-	return __syscall_ret(ret);
+	return ret;
 }