xfs: fix getfsmap userspace memory corruption while setting OF_LAST

At the end of a getfsmap call, we will set FMR_OF_LAST in the last
struct fsmap that was handed in by userspace if we've truly run out of
space mapping record (as opposed to simply running out of space in the
user array).  Unfortunately, fmh_entries is the wrong check for whether
or not we've filled out anything in the user array because the ioctl
provides that fmh_count==0 sets fmh_entries without filling out the user
array.  Therefore we end up writing things into user memory areas that we
weren't given, and kaboom.

Since Christoph amended the getfsmap structure to track the number of
fsmap entries we've actually filled out, use that as part of deciding if
we have to set the OF_LAST flag.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>