Description: bcache: fix potential deadlock problem in btree_gc_coalesce
References:
 https://lkml.org/lkml/2020/4/26/87
Notes:
Bugs:
upstream: needed
4.19-upstream-stable: needed
4.9-upstream-stable: needed
3.16-upstream-stable: needed
sid: needed
4.19-buster-security: needed
4.9-stretch-security: needed
3.16-jessie-security: needed