NFS: Fix a bug in nfs_fscache_release_page()

Not having an fscache cookie is perfectly valid if the user didn't mount
with the fscache option.

This patch fixes http://bugzilla.kernel.org/show_bug.cgi?id=15234

Signed-off-by: Trond Myklebust <[e-mail]>
Acked-by: David Howells <[e-mail]>
Cc: [e-mail]

fs/nfs/fscache.c

···
  */
 int nfs_fscache_release_page(struct page *page, gfp_t gfp)
 {
-	struct nfs_inode *nfsi = NFS_I(page->mapping->host);
-	struct fscache_cookie *cookie = nfsi->fscache;
-
-	BUG_ON(!cookie);
-
 	if (PageFsCache(page)) {
+		struct nfs_inode *nfsi = NFS_I(page->mapping->host);
+		struct fscache_cookie *cookie = nfsi->fscache;
+
+		BUG_ON(!cookie);
 		dfprintk(FSCACHE, "NFS: fscache releasepage (0x%p/0x%p/0x%p)\n",
 			 cookie, page, nfsi);