LU-14286 osd-ldiskfs: fallocate with unwritten extents

LU-14286 osd-ldiskfs: fallocate with unwritten extents

The osd_fallocate() code should typically be allocating unwritten
extents with LDISKFS_GET_BLOCKS_CREATE_UNWRIT_EXT instead of actually
zeroing the blocks on disk with LDISKFS_GET_BLOCKS_CREATE_ZERO.

Writing zeroes during fallocate() is typically slower initially, and
is causing timeouts in sanity test_150e, which is trying to fill up
all OSTs to 90%.  In some cases, zeroing the underlying blocks can
use the underlying storage support for efficient zeroing (WRITE_SAME),
so it may be faster for later use than uninitialized extents that have
to be converted to initialized extents by (possibly) splitting them
into smaller extents and/or zero filling them when they are paritally
being overwritten.

Add a tunable parameter osd-ldiskfs.*.fallocate_zero_blocks to allow
selecting this behavior at runtime.  The default is -1, to disable
fallocate completely (return -EOPNOTSUPP) due to current bugs.

Test-Parameters: testlist=sanityn env=ONLY=16,ONLY_REPEAT=10
Fixes: 72617588ac8c ("LU-14286 osd-ldiskfs: fallocate() should zero new blocks")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ida3692c487fdc8918863fc5c99459caaba17d92e
Reviewed-on: https://review.whamcloud.com/41204
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>