xfs,fstrim: fix to return correct minlen

This patch tries to address two problems:

1) return @minlen we used to trim to
user space.

2) return EINVAL if granularity is larger than
avg size, even most of cases, granularity is small(4K),
but if devices return a lager granularity for some reaons
(testing, bugs etc), fstrim should return failure directly.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>