fs/xattr: Fix handling of AT_FDCWD in setxattrat(2) and getxattrat(2)

Currently, setxattrat(2) and getxattrat(2) are wrongly handling the
calls of the from setxattrat(AF_FDCWD, NULL, AT_EMPTY_PATH, ...) and
fail with -EBADF error instead of operating on CWD. Fix it.

Fixes: 6140be9 ("fs/xattr: add *at family syscalls")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/20250424132246.16822-2-jack@suse.cz
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: jankara

fs/xattr.c

@@ -703,7 +703,7 @@ static int path_setxattrat(int dfd, const char __user *pathname,
 		return error;
 
 	filename = getname_maybe_null(pathname, at_flags);
-	if (!filename) {
+	if (!filename && dfd >= 0) {
 		CLASS(fd, f)(dfd);
 		if (fd_empty(f))
 			error = -EBADF;
@@ -847,7 +847,7 @@ static ssize_t path_getxattrat(int dfd, const char __user *pathname,
 		return error;
 
 	filename = getname_maybe_null(pathname, at_flags);
-	if (!filename) {
+	if (!filename && dfd >= 0) {
 		CLASS(fd, f)(dfd);
 		if (fd_empty(f))
 			return -EBADF;