resize-fs: add helper that checks if the specified fs can do online grow/shrink

There's only one that can do this (btrfs), but let's abstract that fact,
a bit in case the other file systems learn this too one day.

src/shared/resize-fs.c

@@ -119,3 +119,8 @@ uint64_t minimal_size_by_fs_name(const char *name) {
 
         return UINT64_MAX;
 }
+
+/* Returns true for the only fs that can online shrink *and* grow */
+bool fs_can_online_shrink_and_grow(statfs_f_type_t magic) {
+        return magic == (statfs_f_type_t) BTRFS_SUPER_MAGIC;
+}

src/shared/resize-fs.h

@@ -13,3 +13,5 @@ int resize_fs(int fd, uint64_t sz, uint64_t *ret_size);
 
 uint64_t minimal_size_by_fs_magic(statfs_f_type_t magic);
 uint64_t minimal_size_by_fs_name(const char *str);
+
+bool fs_can_online_shrink_and_grow(statfs_f_type_t magic);