DDN-4623 obdclass: fix upcall_cache_get_entry

DDN-4623 obdclass: fix upcall_cache_get_entry

When an entry is found while holding the read lock, we need to
convert to a write lock and find again, to check that entry was
not modified/freed in between.
In this case, the variable indicating an entry was found must be
reset, because we might not find any valid entry after all.

Fixes: 127128bed3 ("LU-16498 obdclass: change uc_lock to rwlock")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I111af4562ac78eeb22102a8a28943e46e30b4019
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53743
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>