dm cache: avoid conflicting remove_mapping() in mq policy

On sparc32, which includes <linux/swap.h> from <asm/pgtable_32.h>:

drivers/md/dm-cache-policy-mq.c:962:13: error: conflicting types for 'remove_mapping'
include/linux/swap.h:285:12: note: previous declaration of 'remove_mapping' was here

As mq_remove_mapping() already exists, and the local remove_mapping() is
used only once, inline it manually to avoid the conflict.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair Kergon <agk@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>