btree_write_buffer: ensure atomic64_sub_return_release availability

btree_write_buffer: ensure atomic64_sub_return_release availability

prior to this patch, on certain platforms (ie: armv7l), compilation fails due
to atomic64_sub_return_release not being defined here. Ensure that the atomics
header is pulled in, and ensure that it is available in all cases, regardless
of whether ATOMIC64_SPINLOCK is defined.

Signed-off-by: Nicholas Sielicki <linux@opensource.nslick.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>