LU-14195 lustre: remove 'fs' from 'struct lvfs_run_ctxt'

LU-14195 lustre: remove 'fs' from 'struct lvfs_run_ctxt'

The code protected by push_ctxt() and pop_ctx() never tries to access
any user-space data, so call set_fs() to KERNEL_DS is not needed.

So remove the 'fs' field and related code.

In linux-5.10 this code fails to compile as set_fs() is deprecated.

Lustre-change: https://review.whamcloud.com/40910
Lustre-commit: de60e7767c0e3ba38f4de37e46328012780b6d19

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Idb2744d656dc4228375b6da54673e38cc1c112f5
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44143
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>