EX-7601 osd: add assert for prepare partial page

EX-7601 osd: add assert for prepare partial page

In the write prep code, we read up any partial pages (pages
which are not completely overwritten by the write) to
prepare them for write.  But for compressed files, we will
have already done this to prepare for decompression.

Add an assert to make sure we catch if this is ever wrong.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I1366b1f5b191a4d581448d692933d562198c3a1f
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53179
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>