LU-18554 llite: improve *_env_thread_* names

LU-18554 llite: improve *_env_thread_* names

There are a number of {vvp, osc}_env_thread_{struct}
helpers, all of which clear the struct (memset( 0 ))
before returning the pointer.

These names are confusing, improve them so I can stop
making the mistake of assuming they just help get the struct
from the env.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I0bdd8857d8ff076aac0872f8355317f800bfcaa5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57427
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>