LU-4619 llite: prevent buffer overflow in fiemap

LU-4619 llite: prevent buffer overflow in fiemap

lov_fiemap() does not take consider its @vallen parameter, which is
the max buffer size the caller can hold for the fiemap extents.

This patch fixes this and limits the max mapped fiemap extent count
to fit in the preallocted buffer.

This patch also fixes a memory out of bound write issue when the
fiemap call is only for detecting the number of existing extent.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I3a786837fbfec7d5b3f848c5d357b28b9967be85
Reviewed-on: http://review.whamcloud.com/9834
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <paf@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>