nilfs2: correct return value kernel-doc descriptions for bmap functions

Similar to the previous patch to fix the ioctl return value descriptions,
this fixes the format of the return value descriptions for bmap (and
btree)-related functions, which was causing the kernel-doc script to emit
a number of warnings.

Link: https://lkml.kernel.org/r/20250110010530.21872-3-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>