ceph: fix endianness bug in frag_tree_split_cmp

sparse says:

    fs/ceph/inode.c:308:36: warning: incorrect type in argument 1 (different base types)
    fs/ceph/inode.c:308:36:    expected unsigned int [unsigned] [usertype] a
    fs/ceph/inode.c:308:36:    got restricted __le32 [usertype] frag
    fs/ceph/inode.c:308:46: warning: incorrect type in argument 2 (different base types)
    fs/ceph/inode.c:308:46:    expected unsigned int [unsigned] [usertype] b
    fs/ceph/inode.c:308:46:    got restricted __le32 [usertype] frag

We need to convert these values to host-endian before calling the
comparator.

Fixes: a407846e

 ("ceph: don't assume frag tree splits in mds reply are sorted")
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>