Add support for `dirsync' mount option. MS_DIRSYNC is on of the
mount(2) mountflags so don't send it as extra mount option to avoid:

 	lxc-start: Invalid argument - failed to mount ...

errors.
Signed-off-by: Sergey S. Kostyliov <rathamahata@gmail.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>