nilfs2: correct return value kernel-doc descriptions for metadata files

Similar to the previous changes to fix return value descriptions, this
fixes the format of the return value descriptions for metadata file
functions other than sufile.

Link: https://lkml.kernel.org/r/20250110010530.21872-5-konishi.ryusuke@gmail.com


Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: "Brian G ." <gissf1@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>