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")

Fixes: 2153e86541 ("LU-17497 obdclass: check upcall incorrect values")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1aee77f78c92c6c571dfe358435a2733cc3ba9d9
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54710
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>