files: Fix restoration of ghost cwd (and root)

When cwd is removed (it can be) we need to collect the respective
file_desc before starting opening any files to properly handle
ghost refcounts. Otherwise we will miss one refcount from the
cwd's on ghost, which in turn will either BUG inside ghost removal,
or will fail the cwd due to the respective dir being removed too
early.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>