LU-1946 mount: Print more helpful message on EBUSY

LU-1946 mount: Print more helpful message on EBUSY

If the mount function returns EBUSY, there is a chance that the
backend filesystem is mounted through the Posix layer. Specifically,
when using a ZFS backend EBUSY will be returned in this use case. As an
aid the to end user, this patch prints a message intended to help the
user diagnose and fix the issue.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: I7bacbb74c1e76efbbea60f76049a9356d601d246
Reviewed-on: http://review.whamcloud.com/3997
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>