libceph: fix off-by-one bug in ceph_encode_filepath()

There is a BUG_ON() call that doesn't account for the single byte
structure version at the start of an encoded filepath in
ceph_encode_filepath().  Fix that.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

include/linux/ceph/decode.h

@@ -151,7 +151,7 @@ static inline void ceph_encode_filepath(void **p, void *end,
 					u64 ino, const char *path)
 {
 	u32 len = path ? strlen(path) : 0;
-	BUG_ON(*p + sizeof(ino) + sizeof(len) + len > end);
+	BUG_ON(*p + 1 + sizeof(ino) + sizeof(len) + len > end);
 	ceph_encode_8(p, 1);
 	ceph_encode_64(p, ino);
 	ceph_encode_32(p, len);