lssu: fix protection period option

Fix a bug of nilfs_get_latest_usage() function which blocks the
protection period specified with -p option from being passed to
nilfs_assess_segment() function and results in incorrect output of
live block counts.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
lssu: display a "p" flag on protected segments

Display a "p" flag in STAT field of the output lines of protected
segments when -l (--latest-usage) option is specfied.  This flag
clarifies which segments are protected by garbage collection at that
moment.

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