This patch adds target_parse_naa_6h_vendor_specific() to address a bug where the
conversion of PRODUCT SERIAL NUMBER to use hex2bin() in target_emulate_evpd_83()
was not doing proper isxdigit() checking.  This conversion of the vpd_unit_serial
configifs attribute is done while generating a VPD=0x83 NAA IEEE Registered
Extended DESIGNATOR format's 100 bits of unique VENDOR SPECIFIC IDENTIFIER +
VENDOR SPECIFIC IDENTIFIER EXTENSION area.

This patch allows vpd_unit_serial (VPD=0x80) and the T10 Vendor ID DESIGNATOR
format (VPD=0x83) to continue to use free-form variable length ASCII values,
and now skips any non hex characters for fixed length NAA IEEE Registered Extended
DESIGNATOR format (VPD=0x83) requring the binary conversion.

This was originally reported by Martin after the v3.1-rc1 change to use hex2bin()
in commit 11650b85 where the use of non hex
characters in vpd_unit_serial generated different values than the original
v3.0 internal hex -> binary code.  This v3.1 change caused a problem with
filesystems who write a NAA DESIGNATOR onto it's ondisk metadata, and this patch
will (again) change existing values to ensure that non hex characters are not
included in the fixed length NAA DESIGNATOR.

Note this patch still expects vpd_unit_serial to be set via existing userspace
methods of uuid generation, and does not do strict formatting via configfs input.

The original bug report and thread can be found here:

NAA breakage
http://www.spinics.net/lists/target-devel/msg00477.html

The v3.1-rc1 formatting of VPD=0x83 w/o this patch:

VPD INQUIRY: Device Identification page
  Designation descriptor number 1, descriptor length: 20
    designator_type: NAA,  code_set: Binary
    associated with the addressed logical unit
      NAA 6, IEEE Company_id: 0x1405
      Vendor Specific Identifier: 0xffde35ebf
      Vendor Specific Identifier Extension: 0x3092f498ffa820f9
      [0x6001405ffde35ebf3092f498ffa820f9]
  Designation descriptor number 2, descriptor length: 56
    designator_type: T10 vendor identification,  code_set: ASCII
    associated with the addressed logical unit
      vendor id: LIO-ORG
      vendor specific: IBLOCK:ffde35ec-3092-4980-a820-917636ca54f1

The v3.1-final formatting of VPD=0x83 w/ this patch:

VPD INQUIRY: Device Identification page
  Designation descriptor number 1, descriptor length: 20
    designator_type: NAA,  code_set: Binary
    associated with the addressed logical unit
      NAA 6, IEEE Company_id: 0x1405
      Vendor Specific Identifier: 0xffde35ec3
      Vendor Specific Identifier Extension: 0x924980a82091763
      [0x6001405ffde35ec30924980a82091763]
  Designation descriptor number 2, descriptor length: 56
    designator_type: T10 vendor identification,  code_set: ASCII
    associated with the addressed logical unit
      vendor id: LIO-ORG
      vendor specific: IBLOCK:ffde35ec-3092-4980-a820-917636ca54f1

(v2: Fix parsing code to dereference + check for string terminator instead
     of null pointer to ensure a zeroed payload for vpd_unit_serial less
     than 100 bits of NAA DESIGNATOR VENDOR SPECIFIC area.  Also, remove
     the unnecessary bitwise assignment)
Reported-by: Martin Svec <martin.svec@zoner.cz>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>