LU-14423 osd: recognize holes in osd_is_mapped()

LU-14423 osd: recognize holes in osd_is_mapped()

ldiskfs_fiemap() can return {0,0,0} for last non-allocated
region.  osd_is_mapped() should be able to recognize and
cache this state.

Fixes: 144b5a65c1 ("LU-7132 osd-ldiskfs: speedup rewrites")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I03883038c2c0ec84754377a442c4947c7e3021a9
Reviewed-on: https://review.whamcloud.com/41481
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>