unix: define PerfBitWriteBackward

This bit, introduced in kernel 4.7, is used to indicate a perf ring buffer will
be written backward [1].
This is mainly used for overwritable perf ring buffer.

Suggested-by: Lorenz Bauer <[email protected]>
Signed-off-by: Francis Laniel <[email protected]>
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9ecda41acb97

Author: Francis Laniel

unix/linux/types.go

@@ -1115,6 +1115,7 @@ const (
 	PerfBitCommExec                      = CBitFieldMaskBit24
 	PerfBitUseClockID                    = CBitFieldMaskBit25
 	PerfBitContextSwitch                 = CBitFieldMaskBit26
+	PerfBitWriteBackward                 = CBitFieldMaskBit27
 )
 
 // generated by: