Add generic logical block size support for UDF.

	Add generic logical block size support for UDF.

	* grub-core/fs/udf.c (GRUB_UDF_LOG2_BLKSIZE): Removed.
	(GRUB_UDF_BLKSZ): Removed.
	(struct grub_udf_data): New field "lbshift" to hold the logical	block
	size of the file system in log2 format. All users updated.
	(sblocklist): Change type to unsigned.
	(grub_udf_mount): Change type of "sblklist" to unsigned.
	Move AVDP search before VRS recognition, because the latter requires
	knowledge of the logical block size, which is detected during the
	former.
	Detect and validate logical block size during AVDP search, adding
	support for block sizes 512, 1024 and 4096.
	Make VRS recognition independent of block size.