pstore: fix build warning for unused return value from sysfs_create_file

fs/pstore/inode.c: In function 'init_pstore_fs':
fs/pstore/inode.c:266: warning: ignoring return value of 'sysfs_create_file', declared with attribute warn_unused_result

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>