inspection: Set fs->device field on swap devices.

For some reason the fs->device field was not set for swap devices.

Apparently this did not matter before (this bug has existed since at
least libguestfs 1.16), but in commit 6ee80b8dac we started to depend
on the fs->device field being non-NULL.  This resulted in segfaults
when inspecting any guest which had a swap device, eg:

 #0  __strcmp_sse42 () at ../sysdeps/x86_64/multiarch/strcmp-sse42.S:164
 #1  0x00007ffff7d740cb in parent_device_already_probed (g=0x703a70,
                        partition=0x6f79b0 "/dev/vda4") at inspect.c:156
 #2  0x00007ffff7d73e00 in guestfs__inspect_os (g=0x703a70) at inspect.c:83
 #3  0x00007ffff7cee11d in guestfs_inspect_os (g=0x703a70) at actions-1.c:397

This commit sets fs->device to the device name.

(Thanks Matthew Booth for first noticing this problem.)

src/inspect-fs.c

@@ -112,6 +112,7 @@ guestfs___check_for_filesystem_on (guestfs_h *g, const char *device,
     if (extend_fses (g) == -1)
       return -1;
     fs = &g->fses[g->nr_fses-1];
+    fs->device = safe_strdup (g, device);
     return 0;
   }