bcachefs: Check gfp_flags correctly in bch2_btree_cache_scan()

bch2_btree_node_mem_alloc() uses memalloc_nofs_save()/GFP_NOFS, but
GFP_NOFS does include __GFP_IO - oops. We used to use GFP_NOIO, but as
we're a filesystem now GFP_NOFS makes more sense now and is looser.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_cache.c

@@ -242,7 +242,7 @@ static unsigned long bch2_btree_cache_scan(struct shrinker *shrink,
 		return SHRINK_STOP;
 
 	/* Return -1 if we can't do anything right now */
-	if (sc->gfp_mask & __GFP_IO)
+	if (sc->gfp_mask & __GFP_FS)
 		mutex_lock(&bc->lock);
 	else if (!mutex_trylock(&bc->lock))
 		return -1;