udf: Make s_block_bitmap standard array

struct udf_bitmap has array of buffer pointers attached to it. The code
unnecessarily used s_block_bitmap as a pointer to the array instead of
the standard trick of using 0 length array in the declaration. Change
that to make code more readable and actually shrink the structure by one
pointer.

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