[PATCH] device-mapper: Fix dm_swap_table error cases

Fix dm_swap_table() __bind error cases: a missing unlock, and EINVAL
preferable to EPERM.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>