fstests: skip AIO-related tests when CONFIG_AIO=n

When running xfstests on a kernel configured with CONFIG_AIO=n, all
AIO-related tests fail, often due to an error similar to the
following:

    error Function not implemented during io_setup

This affected at least the following tests: generic/036,
generic/112, generic/113, generic/198, generic/207, generic/208,
generic/210, generic/211, generic/239, generic/323, generic/427,
xfs/240, xfs/241.

Fix this by enhancing the 'feature' program to allow testing for
asynchronous I/O support, then skipping all AIO-related tests when
AIO is unsupported.

This change is useful because CONFIG_AIO is sometimes disabled to
reduce the kernel's attack surface (e.g. see
https://android-review.googlesource.com/#/c/292158/).

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

common/rc

@@ -1868,7 +1868,24 @@ _require_nonexternal()
 	_notrun "External device testing in progress, skipped this test"
 }
 
+# this test requires that the kernel supports asynchronous I/O
+_require_aio()
+{
+	$here/src/feature -A
+	case $? in
+	0)
+		;;
+	1)
+		_notrun "kernel does not support asynchronous I/O"
+		;;
+	*)
+		_fail "unexpected error testing for asynchronous I/O support"
+		;;
+	esac
+}
+
 # this test requires that a (specified) aio-dio executable exists
+# and that the kernel supports asynchronous I/O.
 # $1 - command (optional)
 #
 _require_aiodio()
@@ -1881,6 +1898,7 @@ _require_aiodio()
         AIO_TEST=src/aio-dio-regress/$1
         [ -x $AIO_TEST ] || _notrun "$AIO_TEST not built"
     fi
+    _require_aio
     _require_odirect
 }