udf: Use kvzalloc() in udf_sb_alloc_bitmap()

Use kvzalloc() in udf_sb_alloc_bitmap() instead of open-coding it.
Size computation wrapped in struct_size() macro to prevent potential
integer overflows.

Link: https://lore.kernel.org/r/20200827221652.64660-1-efremov@linux.com


Signed-off-by: Denis Efremov <efremov@linux.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>