rbd: remove fiemap use from import

On some kernels and filesystems fiemap can be racy and provide
incorrect data even after an fsync. Later we can use SEEK_HOLE and
SEEK_DATA, but for now just detect zero runs like we do with stdin.

Basically this adapts import from stdin to work in the case of a file
or block device, and gets rid of other cruft in the import that used
fiemap.

Fixes: #4388
Backport: bobtail
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
rbd: clean up do_import() a bit

Move declarations above error conditons so we can goto done almost
everywhere. Remove cpp_strerror printing, since it will be done by the
caller.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge branch 'wip-rbd-import' into next

Reviewed-by: Sage Weil <sage.weil@inktank.com>