cachefiles: perform test on s_blocksize when opening cache file.

cachefiles requires that s_blocksize in the cache is not greater than
PAGE_SIZE, and performs the check every time a block is accessed.

Move the test to the place where the file is "opened", where other
file-validity tests are performed.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index fc1056f..c4b8934 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -655,6 +655,8 @@
 			aops = d_backing_inode(object->dentry)->i_mapping->a_ops;
 			if (!aops->bmap)
 				goto check_error;
+			if (object->dentry->d_sb->s_blocksize > PAGE_SIZE)
+				goto check_error;
 
 			object->backer = object->dentry;
 		} else {