LU-13397 lfs: mirror resync to keep sparseness

LU-13397 lfs: mirror resync to keep sparseness

Use SEEK_HOLE/SEEK_DATA in llapi_mirror_resync_many() to
copy just data chunks between components. Holes at the last
component are done with truncate(), holes in other components
are done with fallocate(FALLOC_FL_PUNCH_HOLE). In case of any
punch() error the hole is just copied via read(), i.e. as zeroes

Currently fallocate(FALLOC_FL_PUNCH_HOLE) is not supported yet,
so resync preserves sparseness only for last components

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Id249739c5cd2d1c8a998da3341d326de1a8b8d32
Reviewed-on: https://review.whamcloud.com/41849
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>