ceph: ceph_readpage_to_fscache didn't check if marked

Previously ceph_readpage_to_fscache did not call if page was marked as cached
before calling fscache_write_page resulting in a BUG inside of fscache.

FS-Cache: Assertion failed
------------[ cut here ]------------
kernel BUG at fs/fscache/page.c:874!
invalid opcode: 0000 [#1] SMP
Call Trace:
 [<ffffffffa02e6566>] __ceph_readpage_to_fscache+0x66/0x80 [ceph]
 [<ffffffffa02caf84>] readpage_nounlock+0x124/0x210 [ceph]
 [<ffffffffa02cb08d>] ceph_readpage+0x1d/0x40 [ceph]
 [<ffffffff81126db6>] generic_file_aio_read+0x1f6/0x700
 [<ffffffffa02c6fcc>] ceph_aio_read+0x5fc/0xab0 [ceph]

Signed-off-by: Milosz Tanski <milosz@adfin.com>
Signed-off-by: Sage Weil <sage@inktank.com>

fs/ceph/cache.c

@@ -311,6 +311,9 @@ void ceph_readpage_to_fscache(struct inode *inode, struct page *page)
 	struct ceph_inode_info *ci = ceph_inode(inode);
 	int ret;
 
+	if (!PageFsCache(page))
+		return;
+
 	if (!cache_valid(ci))
 		return;