LU-930 utils: document 'lfs getstripe -N' option

LU-930 utils: document 'lfs getstripe -N' option

Add the '--mirror-count|-N' option to the usage message for
"lfs getstripe" and its man page.

Fixes: 818340364d51 ("LU-11124 utils: add 'lfs getstripe -N' option")
Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I89ffb2661a48336c37b8bdd784e5f9e942cbd798
Reviewed-on: https://review.whamcloud.com/40545
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>