btrfs-util: make sure btrfs_get_block_device_at() works when called without path

src/shared/btrfs-util.c

@@ -101,8 +101,7 @@ int btrfs_get_block_device_at(int dir_fd, const char *path, dev_t *ret) {
         uint64_t id;
         int r;
 
-        assert(dir_fd >= 0 || dir_fd == AT_FDCWD);
-        assert(path);
+        assert(dir_fd >= 0 || IN_SET(dir_fd, AT_FDCWD, XAT_FDROOT));
         assert(ret);
 
         fd = xopenat(dir_fd, path, O_RDONLY|O_CLOEXEC|O_NONBLOCK|O_NOCTTY);

src/shared/btrfs-util.h

@@ -54,7 +54,7 @@ static inline int btrfs_get_block_device(const char *path, dev_t *ret) {
         return btrfs_get_block_device_at(AT_FDCWD, path, ret);
 }
 static inline int btrfs_get_block_device_fd(int fd, dev_t *ret) {
-        return btrfs_get_block_device_at(fd, "", ret);
+        return btrfs_get_block_device_at(fd, NULL, ret);
 }
 
 int btrfs_defrag_fd(int fd);