dm exception store: fix exstore lookup to be case insensitive

dm exception store: fix exstore lookup to be case insensitive

commit f6bd4eb73cdf2a5bf954e497972842f39cabb7e3 upstream.

When snapshots are created using 'p' instead of 'P' as the
exception store type, the device-mapper table loading fails.

This patch makes the code case insensitive as intended and fixes some
regressions reported with device-mapper snapshots.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>