LU-14286 osd-ldiskfs: enable fallocate by default

Enable fallocate on ldiskfs OSTs by default now that the known
problems have been resolved.  The default mode=0 is the standard
"allocate unwritten extents" behavior used by ext4.  This is by
far the fastest for space allocation, but requires the unwritten
extents to be split and/or zeroed when they are overwritten.

The OST fallocate mode=1 can also be set to use "zeroed extents",
which may be handled by "WRITE SAME", "TRIM zeroes data", or
other low-level functionality in the underlying block device.
This is somewhat slower at fallocate() time (especially for very
large allocations), but still avoids sending any data over the
network, avoids runtime overhead from managing the extents.  There
is not yet an FALLOC_FL_* flag to request this behavior from the
client on a per-file basis.

If problems are hit in the field, fallocate can also be disabled
with mode=-1 at runtime or persistently.

   lctl set_param [-P] osd-ldiskfs.*.fallocate_zero_blocks=<mode>

Ensure that all of the tests which currently use fallocate() are
enabling it for test runs, even if the default changes again.

Fixes: 4f18e08099e5 ("LU-14286 osd-ldiskfs: fallocate with unwritten extents")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iefa71c525597d54fc82a3d6de27a50d4d2ce7057
Reviewed-on: https://review.whamcloud.com/41315
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>