bcachefs: Wrap vmap() in memalloc_nofs_save()/restore()

vmalloc() and vmap() don't take GFP_NOFS - this should be pushed further
up the IO path, but for now just doing the simple fix.

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

fs/bcachefs/compress.c

@@ -7,6 +7,7 @@
 #include "super-io.h"
 
 #include <linux/lz4.h>
+#include <linux/sched/mm.h>
 #include <linux/zlib.h>
 #include <linux/zstd.h>
 
@@ -63,7 +64,7 @@ static struct bbuf __bio_map_or_bounce(struct bch_fs *c, struct bio *bio,
 	struct bbuf ret;
 	struct bio_vec bv;
 	struct bvec_iter iter;
-	unsigned nr_pages = 0;
+	unsigned nr_pages = 0, flags;
 	struct page *stack_pages[16];
 	struct page **pages = NULL;
 	void *data;
@@ -103,7 +104,10 @@ static struct bbuf __bio_map_or_bounce(struct bch_fs *c, struct bio *bio,
 	__bio_for_each_segment(bv, bio, iter, start)
 		pages[nr_pages++] = bv.bv_page;
 
+	flags = memalloc_nofs_save();
 	data = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
+	memalloc_nofs_restore(flags);
+
 	if (pages != stack_pages)
 		kfree(pages);