LU-14520 ldlm: replace mdl_mode_t with ldlm_mode

LU-14520 ldlm: replace mdl_mode_t with ldlm_mode

The mdl_mode_t doesn't really provide any value, since it is just a
copy of enum ldlm_mode, so it is more clear to just use ldlm_mode
directly.  Remove mdl_mode_t and conversions to/from ldlm_mode.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I0153fc67f582e51d8a2001054fa58bfae9f2255a
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56761
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>