exofs: fix endian conversion in exofs_sync_fs()

fscb->s_numfiles is an __le64 field so we need to use cpu_to_le64()
to get a little endian 64 bit on big endian systems.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>