LU-13397 llite: support fallocate() on selected mirror

LU-13397 llite: support fallocate() on selected mirror

- add ability to do fallocate() on designated mirror in
  FLR file
- add missing FALLOC_FL_KEEP_SIZE flag to fallocate() call
  in llapi_hole_punch(). It was just not working without
  that flag silently
- add corresponding test_50d in sanity-flr.sh

Fixes: 4126fbb30c ("LU-13397 lfs: mirror resync to keep sparseness")
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I8d700fce904c84458a50650f1d3cb09d23989eba
Reviewed-on: https://review.whamcloud.com/44721
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>