EX-7601 ofd: same-ify preprw_read and preprw_write

EX-7601 ofd: same-ify preprw_read and preprw_write

preprw_read and preprw_write have some sections which are
functionally the same but which have diverged slightly.
(These can't easily be shared between the functions.)

This is a short patch to make them more similar before
adding eof checking to reads.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I7bce912e99e61a4eec4060d6b49d4917894b44c4
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53426
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>