nilfs2: segment usage file cleanups

This will simplify sufile.c by sharing common code which repeatedly
appears in routines updating a segment usage entry; a wrapper function
nilfs_sufile_update() is introduced for the purpose, and counter
modifications are integrated to a new function
nilfs_sufile_mod_counter().

This is a preparation for the successive bugfix patch ("nilfs2: fix
possible mismatch of sufile counters on recovery").

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>