BACKPORT: dm bufio: introduce a global queue

Rename param_spinlock to global_spinlock and introduce a global queue of
all used buffers.  The queue will be used in the following commits.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit af53badc0cd8a511b016a10c0cccf916692f1fc2)

Bug: 136247322
Change-Id: I00fac757cdb8dadf23ed6140076dc5217468d137
Signed-off-by: Martijn Coenen <maco@android.com>