LU-14565 ofd: Do not rely on tgd_blockbit

LU-14565 ofd: Do not rely on tgd_blockbit

tgd_blockbit is recordsize bits set during mkfs.
This once set does not change. However, 'zfs set'
can be used to change the OST blocksize. Instead
of using cached value of 'tgd_blockbit' always
calculate the blocksize bits which may have
changed.

Test-case: sanity/104c added.

Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: Icc100cca0d5ae492c41d60f0bf97512450f796bc
Reviewed-on: https://review.whamcloud.com/43154
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>