dm: remove dm_mutex after bkl conversion

dm: remove dm_mutex after bkl conversion

This patch replaces dm_mutex with _minor_lock in dm_blk_close()
and then removes it.

During the BKL conversion, commit 5ba94a609fe52e961af095ebc04a3faeb4f36b49
(block: push down BKL into .open and .release) pushed lock_kernel()
down into dm_blk_open/close calls.
Commit f525a846c435ad400506edd5216e88b775c3e63f
(block: autoconvert trivial BKL users to private mutex) converted it to a
local mutex, but _minor_lock is sufficient.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>