udf: Fortify loading of sparing table

commit 1df2ae31c724e57be9d7ac00d78db8a5dabdd050 upstream.

Add sanity checks when loading sparing table from disk to avoid accessing
unallocated memory or writing to it.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>