UBI: allocate memory with GFP_NOFS

Use GFP_NOFS flag when allocating memory on I/O path, because otherwise
we may deadlock the filesystem which works on top of us. We observed
the deadlocks with UBIFS. Example:

VFS->FS lock a lock->UBI->kmalloc()->VFS writeback->FS locks the same
lock again.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/io.c b/drivers/mtd/ubi/io.c
index 9284763..ba5bc4a 100644
--- a/drivers/mtd/ubi/io.c
+++ b/drivers/mtd/ubi/io.c
@@ -1099,7 +1099,7 @@
 	uint32_t crc, hdr_crc;
 	struct ubi_ec_hdr *ec_hdr;
 
-	ec_hdr = kzalloc(ubi->ec_hdr_alsize, GFP_KERNEL);
+	ec_hdr = kzalloc(ubi->ec_hdr_alsize, GFP_NOFS);
 	if (!ec_hdr)
 		return -ENOMEM;
 
@@ -1179,7 +1179,7 @@
 	struct ubi_vid_hdr *vid_hdr;
 	void *p;
 
-	vid_hdr = ubi_zalloc_vid_hdr(ubi);
+	vid_hdr = ubi_zalloc_vid_hdr(ubi, GFP_NOFS);
 	if (!vid_hdr)
 		return -ENOMEM;