LU-13485 libcfs: FIELD_SIZEOF macro removed

LU-13485 libcfs: FIELD_SIZEOF macro removed

Linux v4.15-rc2-5-g4229a470175b introduced sizeof_field() macro
Linux v5.5-rc4-1-g1f07dcc459d5 removed FIELD_SIZEOF() macro

Provide a sizeof_field() macro in terms of FIELD_SIZEOF()
when sizeof_field() is not provided.

Test-Parameters: trivial
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I48ca9abb931d58919d788199e5089984c9e854dd
Reviewed-on: https://review.whamcloud.com/39710
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>