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>

fs/xfs/xfs_discard.c

@@ -172,6 +172,8 @@ xfs_ioc_trim(
 	if (copy_from_user(&range, urange, sizeof(range)))
 		return -EFAULT;
 
+	range.minlen = max_t(u64, granularity, range.minlen);
+	minlen = BTOBB(range.minlen);
 	/*
 	 * Truncating down the len isn't actually quite correct, but using
 	 * BBTOB would mean we trivially get overflows for values
@@ -186,7 +188,6 @@ xfs_ioc_trim(
 
 	start = BTOBB(range.start);
 	end = start + BTOBBT(range.len) - 1;
-	minlen = BTOBB(max_t(u64, granularity, range.minlen));
 
 	if (end > XFS_FSB_TO_BB(mp, mp->m_sb.sb_dblocks) - 1)
 		end = XFS_FSB_TO_BB(mp, mp->m_sb.sb_dblocks)- 1;