LU-12885 mds: add enums for MDS_OPEN flags

LU-12885 mds: add enums for MDS_OPEN flags

This patch is first of the series of patch that separates
kernel open flags from MDS open flags

The first step is to add enum mds_open_flags to the code to
make it easier to follow the logic. Rename it_flags to
it_open_flags and use enum mds_open_flags in the code so it
is clear that MDS_OPEN flags are being used.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I933a6e6102f947a9276cb6bf03826fd4a53ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/36469
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>