LU-12904 gss: struct cache_detail readers changed to writers

LU-12904 gss: struct cache_detail readers changed to writers

Linux 5.3 changed struct cache_detail readers to writers
SUNRPC: Track writers of the 'channel' file to improve ...

kernel-commit: 64a38e840ce5940253208eaba40265c73decc4ee

This patch is back-ported from the following one:
Lustre-commit: 81fb8bc7d214394bbc504379a5a84441e871b60a
Lustre-change: https://review.whamcloud.com/36580

Test-Parameters: trivial
Cray-bug-id: LUS-8042
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I7750303937cd6fc560e458efa79f25e521fefec7
Reviewed-on: https://review.whamcloud.com/38193
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>