LU-13720 lfs: resync file if needed before mirror_split()

LU-13720 lfs: resync file if needed before mirror_split()

If "lfs mirror delete" is called on a file to delete the only
non-stale mirror of the file, it makes sense from a usability
point of view to automatically resync the stale mirror(s) and
then remove the mirror requested by the user, rather than
returning an error to the user.

Also, sanity-flr.sh test_44a/b and test_211 are modified to verify
this patch. Man pages lfs-mirror-delete.1 and lfs-mirror-split.1
are updated to reflect this change.

Test-Parameters: testlist=sanity-flr env=ONLY="44,211"

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: Ice5ef893910e744d235dcb07f1049dfeede65858
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55978
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Zhenyu Xu <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>