basic: Remove assertions from missing_fs.h

These assertions impose an include order between <linux/fs.h> and
"missing_fs.h", specifically <linux/fs.h> can't be included before
"missing_fs.h". This makes automated include refactoring very painful,
so let's get rid of these assertions and instead assume that linux/fs.h
does the right thing.

src/shared/btrfs-util.c

@@ -3,6 +3,8 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
+#include <linux/btrfs.h>
+#include <linux/btrfs_tree.h>
 #include <linux/loop.h>
 #include <linux/magic.h>
 #include <stddef.h>

src/shared/copy.c

@@ -2,6 +2,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <linux/btrfs.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>

src/shared/resize-fs.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <linux/btrfs.h>
 #include <linux/magic.h>
 #include <sys/ioctl.h>
 #include <sys/vfs.h>