introduce __fsname_is_auto(name, force_auto)

Simple preparation to simplify the review of the next patch. Turn
find_fstype_by_name(name) into __find_fstype_by_name(name, force_auto)
and reimplement find_fstype_by_name() as a trivial wrapper on top.

This allows "restore" to specify that this particular fsname was treated
as FSTYPE__AUTO by "dump".

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>