UBIFS: do not print scary memory allocation warnings

Bulk-read allocates a lot of memory with 'kmalloc()', and when it
is/gets fragmented 'kmalloc()' fails with a scarry warning. But
because bulk-read is just an optimization, UBIFS keeps working fine.
Supress the warning by passing __GFP_NOWARN option to 'kmalloc()'.

This patch also introduces a macro for the magic 128KiB constant.
This is just neater.

Note, this is not really fixes the problem we had, but just hides
the warnings. The further patches fix the problem.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
index a7bd32f..06ba51e 100644
--- a/fs/ubifs/ubifs.h
+++ b/fs/ubifs/ubifs.h
@@ -753,7 +753,7 @@
 };
 
 /**
- * struct bu_info - bulk-read information
+ * struct bu_info - bulk-read information.
  * @key: first data node key
  * @zbranch: zbranches of data nodes to bulk read
  * @buf: buffer to read into