ext4: Adjust minlen with discard_granularity in the FITRIM ioctl

Discard granularity tells us the minimum size of extent that can be
discarded by the device.  If the user supplies a minimum extent that
should be discarded (range.minlen) which is smaller than the discard
granularity, increase minlen to the discard granularity, since there's
no point submitting trim requests that the device will reject anyway.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ioctl.c

@@ -348,6 +348,8 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		    sizeof(range)))
 			return -EFAULT;
 
+		range.minlen = max((unsigned int)range.minlen,
+				   q->limits.discard_granularity);
 		ret = ext4_trim_fs(sb, &range);
 		if (ret < 0)
 			return ret;