udf: Remove BKL from free space counting functions

udf_count_free_bitmap() does not need BKL because bitmaps are in a fixed
place on disk and so we can count set bits without serialization.
udf_count_free_table() is now protected by s_alloc_mutex instead of BKL
to get a consistent view of free space extents.

Signed-off-by: Jan Kara <jack@suse.cz>