lib/deploy: Add 1M margin in early prune space check

There are a few things the estimator doesn't account for, e.g. writing
the new BLS entries. Rather than trying to perfect it (since I think we
should change approach entirely -- see previous commit message), just
add a 1M margin to the space check.

src/libostree/ostree-sysroot-deploy.c

@@ -2541,6 +2541,10 @@ dfd_fallocate_check (int dfd, off_t len, gboolean *out_passed, GError **error)
   if (!glnx_open_tmpfile_linkable_at (dfd, ".", O_WRONLY | O_CLOEXEC, &tmpf, error))
     return FALSE;
 
+  /* add 1M as a large safety margin to account for any files also written to
+   * /boot that aren't counted, like the BLS entry file */
+  len += 1 << 20;
+
   *out_passed = TRUE;
   /* There's glnx_try_fallocate, but not with the same error semantics. */
   if (TEMP_FAILURE_RETRY (fallocate (tmpf.fd, 0, 0, len)) < 0)