LU-16446 utils: specify total count for mirror extend

LU-16446 utils: specify total count for mirror extend

The 'lfs mirror extend -N' can now be used to specify the total
number of mirrors to create on a file by using '=' infront of the
COUNT:
    'lfs mirror extend -N=TOTAL_COUNT'
or
    'lfs mirror extend --mirror-count==TOTAL_COUNT'

This is a no-op if the specified number of mirrors already exists on
that file.

Signed-off-by: Frederick Dilger <fdilger@whamcloud.com>
Change-Id: I76fc416b4dc2c37edf99926bae9a8d42167a49ab
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55867
Reviewed-by: Alexandre Ioffe <aioffe@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>