This patch rearranges the snapshot_map code so that the functional changes in
subsequent patches are clearer.

The only functional change is to replace the existing read lock with a write
lock which the next patch needs.
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>