Revert "lib/genalloc.c: make the avail variable an atomic_long_t"

Revert "lib/genalloc.c: make the avail variable an atomic_long_t"

This reverts commit a77c116075936faa6e8b8622e22acc9cbf47fe9e which is
commit 36a3d1dd4e16bcd0d2ddfb4a2ec7092f0ae0d931 upsream. It converts the
`avail' field in struct gen_pool_chunk' to an atomic_long, causing an
ABI breaks.

Gbp-Pq: Topic debian
Gbp-Pq: Name revert-lib-genalloc.c-make-the-avail-variable-an-ato.patch