btrfs: discard: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in header file
related to Btrfs File System support.  For C header files
Documentation/process/license-rules.rst mandates C-like comments
(opposed to C source files where C++ style should be used).

Changes made by using a script provided by Joe Perches here:
https://lkml.org/lkml/2019/2/7/46

.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>