netfs: Pass flags through to grab_cache_page_write_begin()

In netfs_write_begin(), pass the AOP flags through to
grab_cache_page_write_begin() so that a request to use GFP_NOFS is
honoured.

Fixes: e1b1240c ("netfs: Add write_begin helper")
Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
cc: linux-mm@kvack.org
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/162090295383.3165945.13595101698295243662.stgit@warthog.procyon.org.uk # v1

fs/netfs/read_helper.c

@@ -1068,7 +1068,7 @@ int netfs_write_begin(struct file *file, struct address_space *mapping,
 	DEFINE_READAHEAD(ractl, file, NULL, mapping, index);
 
 retry:
-	page = grab_cache_page_write_begin(mapping, index, 0);
+	page = grab_cache_page_write_begin(mapping, index, flags);
 	if (!page)
 		return -ENOMEM;