This is something we need to do more widely: instead of bothering with
GFP_NOIO/GFP_NOFS, if we need to allocate memory while holding locks:

 - first attempt the allocation with GFP_NOWAIT
 - if that fails, drop btree locks with bch2_trans_unlock(), then
   retry with GFP_KERNEL.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>