LU-14286 osd-ldiskfs: don't read unwritten blocks

LU-14286 osd-ldiskfs: don't read unwritten blocks

which were allocated using fallocate(2), instead fill
the pages with zeroes.

Add a test to verify that fallocated blocks read as zeroes.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I9c0c90f93fd33f26f834144e225b2643cf9fffb7
Reviewed-on: https://review.whamcloud.com/41216
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>