exofs: readpage_strip: Add a BUG_ON to check for PageLocked(page)

readpage_strip can be called from several code paths all of which
require that the page be locked before any operations are carried
out.

Since we export the exofs_readpage callback to the VFS, add a
BUG_ON to check for PageLocked(page) to make sure that this
understanding is never compromised.
Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>

fs/exofs/inode.c

@@ -389,6 +389,8 @@ static int readpage_strip(void *data, struct page *page)
 	size_t len;
 	int ret;
 
+	BUG_ON(!PageLocked(page));
+
 	/* FIXME: Just for debugging, will be removed */
 	if (PageUptodate(page))
 		EXOFS_ERR("PageUptodate(0x%lx, 0x%lx)\n", pcol->inode->i_ino,