unshare: remove get_mnt_ino() check in bind_ns_files_from_child()

unshare: remove get_mnt_ino() check in bind_ns_files_from_child()

get_mnt_ino() was originally introduced in c84f2590 where it was used
in a loop to wait for the parent process to unshare the mount namespace
before binding the namespace in its child.

The parent and child processes are now synchronised with eventfd, so
remove this vestigial check and the now-unused get_mnt_ino() function.
This allows bind_ns_files_from_child() to be used even when the mount
namespace isn't amongst the namespaces being unshared.

Signed-off-by: Chris Webb <chris@arachsys.com>