We're still seeing OOM issues caused by the btree node cache shrinker
not sufficiently freeing memory: thus, this patch changes the shrinker
to not exit if __GFP_FS was not supplied.

Instead, tweak btree node memory allocation so that we never invoke
memory reclaim while holding the btree node cache lock.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>