LU-12580 lov: fix out of bound usercopy

LU-12580 lov: fix out of bound usercopy

When handling ioctl LL_IOC_LOV_GETSTRIPE, the user
could pass a limited buffer which is bigger than
lov_comp_md_size(), it will crash the client because
we are doing the usercopy with the user provided buffer
size.

Make sure the copy works, also for the PFL file,
we should only copy the chosen component.

Change-Id: I92bcf6d7b7f7a4387a9936a0b58332e50a88e542
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/37469
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>