btrfs: Return EINVAL when length to trim is less than FSB

Currently if len argument in btrfs_ioctl_fitrim() is smaller than
one FSB we will continue and finally return 0 bytes discarded.
However if the length to discard is smaller then file system block
we should really return EINVAL.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>

fs/btrfs/ioctl.c

@@ -343,7 +343,8 @@ static noinline int btrfs_ioctl_fitrim(struct file *file, void __user *arg)
 		return -EOPNOTSUPP;
 	if (copy_from_user(&range, arg, sizeof(range)))
 		return -EFAULT;
-	if (range.start > total_bytes)
+	if (range.start > total_bytes ||
+	    range.len < fs_info->sb->s_blocksize)
 		return -EINVAL;
 
 	range.len = min(range.len, total_bytes - range.start);