UPSTREAM: init: export name_to_dev_t and mark name argument as const

DM will switch its device lookup code to using name_to_dev_t() so it
must be exported.  Also, the @name argument should be marked const.

Signed-off-by: Dan Ehrenberg <dehrenberg@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

Change-Id: If997bf85d80395f0bdb8b84e669deecf8def075c
[AmitP: Fix undefined "name_to_dev_t" error when building
        dm-android-verity driver as a module in android-3.18]
(cherry picked from commit e6e20a7a5f3f49bfee518d5c6849107398d83912)
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>