xfs: Fix UBSAN null-ptr-deref in xfs_sysfs_init

xfs: Fix UBSAN null-ptr-deref in xfs_sysfs_init

If xfs_sysfs_init is called with parent_kobj == NULL, UBSAN
shows the following warning:

  UBSAN: null-ptr-deref in ./fs/xfs/xfs_sysfs.h:37:23
  member access within null pointer of type 'struct xfs_kobj'
  Call Trace:
   dump_stack+0x10e/0x195
   ubsan_type_mismatch_common+0x241/0x280
   __ubsan_handle_type_mismatch_v1+0x32/0x40
   init_xfs_fs+0x12b/0x28f
   do_one_initcall+0xdd/0x1d0
   do_initcall_level+0x151/0x1b6
   do_initcalls+0x50/0x8f
   do_basic_setup+0x29/0x2b
   kernel_init_freeable+0x19f/0x20b
   kernel_init+0x11/0x1e0
   ret_from_fork+0x22/0x30

Fix it by checking parent_kobj before the code accesses its member.

Signed-off-by: Eiichi Tsukata <devel@etsukata.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: minor whitespace edits]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>