LU-17718 obdclass: potential string overflow upcall_cache.c

LU-17718 obdclass: potential string overflow upcall_cache.c

Use strncpy() in upcall_cache_set_upcall() to quiet Coverity warning.
And reorganize the function so that the code flow is more linear in
the success case.

CoverityID: 424705: ("String overflow")

Lustre-change: https://review.whamcloud.com/54710
Lustre-commit: 7869bb320e735547410a7d3e31061b9044389c53

Fixes: a462a119ec ("LU-17497 obdclass: check upcall incorrect values")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1aee77f78c92c6c571dfe358435a2733cc3ba9d9
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/55314
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>