ceph: handle -EAGAIN returned by ceph_update_writeable_page()

when ceph_update_writeable_page() return -EAGAIN, caller should
lock the page and call ceph_update_writeable_page() again.
Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/addr.c

@@ -1475,12 +1475,13 @@ static int ceph_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	/* Update time before taking page lock */
 	file_update_time(vma->vm_file);
 
+	do {
 		lock_page(page);
 
 		if ((off > size) || (page->mapping != inode->i_mapping)) {
 			unlock_page(page);
 			ret = VM_FAULT_NOPAGE;
-		goto out;
+			break;
 		}
 
 		ret = ceph_update_writeable_page(vma->vm_file, off, len, page);
@@ -1489,7 +1490,8 @@ static int ceph_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 			set_page_dirty(page);
 			ret = VM_FAULT_LOCKED;
 		}
-out:
+	} while (ret == -EAGAIN);
+
 	if (ret == VM_FAULT_LOCKED ||
 	    ci->i_inline_version != CEPH_INLINE_NONE) {
 		int dirty;