EX-8369 ldiskfs: mballoc to store group in pa

EX-8369 ldiskfs: mballoc to store group in pa

mballoc should store and then use group number from pa,
not calculate it from pa_pstart which change and point to
a subsequent group in the case of full pa consumption.

Fixes: f36eda6a1e ("LU-10026 osd-ldiskfs: use preallocation for dense writes")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I5e3c5c7e51fe9688f4d847a9ea0591486d8975d8
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52754
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>