nilfs2: localize highmem mapping for checkpoint creation within cpfile

nilfs2: localize highmem mapping for checkpoint creation within cpfile

In order to convert kmap() used in cpfile to kmap_local, first move the
checkpoint creation routine, which is one of the places where kmap is
used, to the cpfile side and make the page mapping local and temporary.
And use kmap_local instead of kmap to access the checkpoint entry page
(and header block page) when generating a checkpoint.

Link: https://lkml.kernel.org/r/20240122140202.6950-12-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>