scsi: lpfc: Fix gcc -Wstringop-overread warning

gcc-11 warns about an strnlen with a length larger than the size of the
passed buffer:

drivers/scsi/lpfc/lpfc_attr.c: In function 'lpfc_nvme_info_show':
drivers/scsi/lpfc/lpfc_attr.c:518:25: error: 'strnlen' specified bound 4095 exceeds source size 24 [-Werror=stringop-overread]
  518 |                         strnlen(LPFC_NVME_INFO_MORE_STR, PAGE_SIZE - 1)
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In this case, the code is entirely valid, as the string is properly
terminated, and the size argument is only there out of extra caution in
case it exceeds a page.

This cannot really happen here, so just simplify it to a sizeof().

Link: https://lore.kernel.org/r/20210322160253.4032422-10-arnd@kernel.org
Fixes: afff0d23

 ("scsi: lpfc: Add Buffer overflow check, when nvme_info larger than PAGE_SIZE")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>