nilfs2: fix missing initial short descriptions of kernel-doc comments

nilfs2: fix missing initial short descriptions of kernel-doc comments

Update some kernel-doc comments that are missing the initial short
description and fix the following warnings output by the kernel-doc
script:

 fs/nilfs2/bmap.c:353: warning: missing initial short description on line:
  * nilfs_bmap_lookup_dirty_buffers -
 fs/nilfs2/cpfile.c:708: warning: missing initial short description on line:
  * nilfs_cpfile_delete_checkpoint -
 fs/nilfs2/cpfile.c:972: warning: missing initial short description on line:
  * nilfs_cpfile_is_snapshot -
 fs/nilfs2/dat.c:275: warning: missing initial short description on line:
  * nilfs_dat_mark_dirty -
 fs/nilfs2/sufile.c:844: warning: missing initial short description on line:
  * nilfs_sufile_get_suinfo -

Link: https://lkml.kernel.org/r/20240816074319.3253-9-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>