bcachefs: Fix deadlock

This fixes two deadlocks:

1.pcpu_alloc_mutex involved one as pointed by syzbot[1]
2.recursion deadlock.

The root cause is that we hold the bc lock during alloc_percpu, fix it
by following the pattern used by __btree_node_mem_alloc().

[1] https://lore.kernel.org/all/66f97d9a.050a0220.6bad9.001d.GAE@google.com/T/



Reported-by:  <syzbot+fe63f377148a6371a9db@syzkaller.appspotmail.com>
Tested-by:  <syzbot+fe63f377148a6371a9db@syzkaller.appspotmail.com>
Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>