nilfs2: remove spaces before tabs

This kills the following checkpatch warnings:

 WARNING: please, no space before tabs
 #74: FILE: segment.h:74:
 +^Iunsigned ^I^Iflags;$

 WARNING: please, no space before tabs
 #35: FILE: segbuf.c:35:
 +^Iint ^I^I^Istart, end; /* The region to be submitted */$
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>