scsi: target: core: Fix invalid memory access

nr_attrs should start counting from zero, otherwise we will end up
dereferencing an invalid memory address.

$ targetcli /loopback create

 general protection fault
 RIP: 0010:configfs_create_file+0x12/0x70
 Call Trace:
  <TASK>
  configfs_attach_item.part.0+0x5f/0x150
  configfs_attach_group.isra.0+0x49/0x120
  configfs_mkdir+0x24f/0x4d0
  vfs_mkdir+0x192/0x240
  do_mkdirat+0x131/0x160
  __x64_sys_mkdir+0x48/0x70
  do_syscall_64+0x5c/0x90

Fixes: 31177b74

 ("scsi: target: core: Add RTPI attribute for target port")
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Link: https://lore.kernel.org/r/20230407130033.556644-1-mlombard@redhat.com
Acked-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>