LU-13765 osd-ldiskfs: Rename dt_declare_falloc to dt_declare_fallocate

LU-13765 osd-ldiskfs: Rename dt_declare_falloc to dt_declare_fallocate

This patch is the follow up of the patch: 93f700ca24
(LU-13765 osd-ldiskfs: Extend credit correctly for fallocate) and
it makes these changes:

01. Rename dt_declare_falloc() to dt_declare_fallocate()
for better readability.

02. Removes fallocate mode check under osd_fallocate()
as mode check is already done under declare phase.

03. Minor space/tabs changes

Test-Parameters: trivial testlist=sanity ostsizegb=12 env=ONLY="150e"
Test-Parameters: testlist=sanity-quota
Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Change-Id: If911a59a9c944e660e9926f4c436a4aeb2919284
Reviewed-on: https://review.whamcloud.com/40509
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>