mount options: fix hpfs

Add a .show_options super operation to hpfs.

Use generic_show_options() and save the complete option string in
hpfs_fill_super() and hpfs_remount_fs().

Also add a small fix: hpfs_remount_fs() should return -EINVAL on
error, instead of 1, which is not an error value.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>