nilfs2: pass remount flag to parse_options

This adds is_remount argument to the parse_options() function that
obtains mount options from strings.

Previously, parse_options did not distinguish context whether it's
called for a new mount or remount, so the caller needed additional
verifications outside the function.

This allows parse_options to verify options and print messages
depending on the context.

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