bcachefs: Check if stuck in journal_res_get()

Like how we already do when the allocator seems to be stuck, check if
we're waiting too long for a journal reservation and print some debug
info.

This is specifically to track down
https://github.com/koverstreet/bcachefs/issues/656



which is showing up in userspace where we don't have sysfs/debugfs to
get the journal debug info.

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