bcachefs: Create allocator threads when allocating filesystem

bcachefs: Create allocator threads when allocating filesystem

We're seeing failures to mount because of a failure to start the
allocator threads, which currently happens fairly late in the mount
process, after walking all metadata, and kthread_create() fails if
something has tried to kill the mount process, which is probably not
what we want.

This patch avoids this issue by creating, but not starting, the
allocator threads when we preallocate all of our other in memory data
structures.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>