swap: use separate priority list for available swap_infos

Originally get_swap_page() started iterating through the singly-linked
list of swap_info_structs using swap_list.next or highest_priority_index,
which both were intended to point to the highest priority active swap
target that was not full.  The first patch in this series changed the
singly-linked list to a doubly-linked list, and removed the logic to start
at the highest priority non-full entry; it starts scanning at the highest
priority entry each time, even if the entry is full.

Add a new list, also priority ordered, to track only swap_info_structs
that are available, i.e. active and not full.  Use a new spinlock so that
entries can be added/removed outside of get_swap_page; that wasn't possible
previously because the main list is protected by swap_lock, which can't be
taken when holding a swap_info_struct->lock because of locking order.
The get_swap_page() logic now does not need to hold the swap_lock, and it
iterates only through swap_info_structs that are available.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>