Only sync the filesystem containing the initramfs

This avoids update-initramfs taking an unnecessarily long time if there
is lots of activity on a different filesystem, or even hanging in cases
such as a stale NFS mount.

Based on a patch by Jukka Tastula <poltsy@gmail.com>.

Closes: #882380
LP: #1667512
Signed-off-by: Colin Watson <cjwatson@debian.org>