unix: add new fscrypt declarations from Linux v5.4

Create Go bindings for the updated fscrypt UAPI from Linux v5.4:

- Various new ioctls.  We use the regex FS_IOC_.*ENCRYPTION to match
  both the new and existing fscrypt ioctls.

- Various new structures for passing to the ioctls

- Various new constants named like FSCRYPT_*

- FscryptPolicy was renamed to FscryptPolicyV1.  (But the old name is
  still available for source compatibility.)

- The existing fscrypt-related constants named like FS_* (besides the
  ioctls) were renamed to FSCRYPT_*.  (But the old names are still
  available for source compatibility.)

For reference, see fscrypt.h:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/fscrypt.h?h=v5.4

Fixes golang/go#35856

Change-Id: I48ddd37cfa7c1e0e7eb227baa6f220c848ddc880
Reviewed-on: https://go-review.googlesource.com/c/sys/+/208898
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/mkerrors.sh

@@ -197,6 +197,7 @@ struct ltchars {
 #include <linux/fanotify.h>
 #include <linux/filter.h>
 #include <linux/fs.h>
+#include <linux/fscrypt.h>
 #include <linux/genetlink.h>
 #include <linux/hdreg.h>
 #include <linux/icmpv6.h>
@@ -496,7 +497,9 @@ ccflags="$@"
 		$2 ~ /^CAN_/ ||
 		$2 ~ /^CAP_/ ||
 		$2 ~ /^ALG_/ ||
-		$2 ~ /^FS_(POLICY_FLAGS|KEY_DESC|ENCRYPTION_MODE|[A-Z0-9_]+_KEY_SIZE|IOC_(GET|SET)_ENCRYPTION)/ ||
+		$2 ~ /^FS_(POLICY_FLAGS|KEY_DESC|ENCRYPTION_MODE|[A-Z0-9_]+_KEY_SIZE)/ ||
+		$2 ~ /^FS_IOC_.*ENCRYPTION/ ||
+		$2 ~ /^FSCRYPT_/ ||
 		$2 ~ /^GRND_/ ||
 		$2 ~ /^RND/ ||
 		$2 ~ /^KEY_(SPEC|REQKEY_DEFL)_/ ||