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>
(cherry picked from commit 3091283895e8ffa3e4bda13399318a6e720d498f)