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