Simple functions to record and restore bio state so we can resubmit a bio
that returned an error.

DM multipath (following shortly) uses this.

This patch has it private to device-mapper: is it any use elsewhere?
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>