LU-15761 obdclass: fix locking in llog_cat_refresh()

LU-15761 obdclass: fix locking in llog_cat_refresh()

the patch fixes two problems:
1) pairing up_write() should be used with cathandle
2) llog_read_header() manipulates loghandle's internal
   structures (header, last_idx, etc) which are supposed
   to stay consistent from another user's point of view
   (like llog_add_rec())

Fixes: 71f409c9b31b ("LU-11418 llog: refresh remote llog upon -ESTALE")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ib86e10a925b541d02c22d74e6ddbc4368345ac11
Reviewed-on: https://review.whamcloud.com/47185
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>