init: do not fork before doing (u)mount_all()

A fork() was historically added in case of fs_mgr crashing or leaking
memory, but this should not be the case with fs_mgr, and a fork() only
hides any such problem, instead of allowing us to address it
directly.

Test: boot
Change-Id: If7ee4807757048258a6ea9a79a24cebbacc530cc