ceph: Don't use ceph-sync-mode for synchronous-fs.

Sending reads and writes through the sync read/write paths bypasses the
page cache, which is not expected or generally a good idea.  Removing
the write check is safe as there is a conditional vfs_fsync_range() later
in ceph_aio_write that already checks for the same flag (via
IS_SYNC(inode)).

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Reviewed-by: Sage Weil <sage@inktank.com>