udf: Use correct partition reference number for metadata

UDF/OSTA terminology is confusing. Partition Numbers (PNs) are arbitrary
16-bit values, one for each physical partition in the volume.  Partition
Reference Numbers (PRNs) are indices into the the Partition Map Table
and do not necessarily equal the PN of the mapped partition.

The current metadata code mistakenly uses the PN instead of the PRN when
mapping metadata blocks to physical/sparable blocks.  Windows-created
UDF 2.5 discs for some reason use large, arbitrary PNs, resulting in
mount failure and KASAN read warnings in udf_read_inode().

For example, a NetBSD UDF 2.5 partition might look like this:

PRN PN Type
--- -- ----
  0  0 Sparable
  1  0 Metadata

Since PRN == PN, we are fine.

But Windows could gives us:

PRN PN   Type
--- ---- ----
  0 8192 Sparable
  1 8192 Metadata

So udf_read_inode() will start out by checking the partition length in
sbi->s_partmaps[8192], which is obviously out of bounds.

Fix this by creating a new field (s_phy...