sysfs: remove s_sibling hacks

s_sibling was used for three different purposes:
1) as a linked list of entries in the directory
2) as a linked list of entries to be deleted
3) as a pointer to "struct completion"

This patch removes the hack and introduces new union u which
holds pointers for cases 2) and 3).

This change is needed for the following patch that removes s_sibling at all
and replaces it with a rb tree.
Signed-off-by: NMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>