LU-13403 utils: mirror-count not required for mirror extend

LU-13403 utils: mirror-count not required for mirror extend

If [--mirror-count|-N] is not specified for 'lfs mirror extend', '-N'
will be added to the option arguments before lfs_setstripe_internal()
is called.

The lustre manual states for 'lfs mirror extend':
    "The mirror_count argument is optional and default to 1 if it is
     not specified."
Which can be interpretend as [--mirror-count|-N] does not need to be
specified rather than the MIRROR_COUNT being optional.

It also makes sense that someone who is using 'lfs mirror extend' in
fact intends to extend the mirror count, so it would make sense to
have a default for mirror-count without it having to be specified.

Signed-off-by: Frederick Dilger <fdilger@whamcloud.com>
Change-Id: I01bea6cec71fbf61c617cf27a52d7fb24fd4b06d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55677
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>