All calls to nilfs_cpfile_get_checkpoint() and
nilfs_cpfile_put_checkpoint() that call kmap() and kunmap() separately are
now gone, so remove these methods.

Link: https://lkml.kernel.org/r/20240122140202.6950-15-konishi.ryusuke@gmail.comSigned-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>