Linux: fall back to FUSE when dmsetup is not installed (#1898)

Without dmsetup, MountVolumeNative fails with "dmsetup not found in
system directories" instead of using the FUSE path, so on systems that
do not ship device-mapper tools (for example OpenWrt) every mount and
the filesystem formatting step of --create need an explicit
--mount-options=nokernelcrypto. Treat a missing dmsetup like the other
cases where kernel crypto cannot be used.

Signed-off-by: Ville Takio <ville+git@takio.fi>

src/Core/Unix/Linux/CoreLinux.cpp

@@ -502,6 +502,12 @@ namespace VeraCrypt
 		if (!SystemInfo::IsVersionAtLeast (2, 6, xts ? 24 : 20))
 			throw NotApplicable (SRC_POS);
 
+		// Without dmsetup, kernel crypto cannot be used: fall back to FUSE
+		// instead of failing, as when nokernelcrypto was given.
+		string dmsetupError;
+		if (Process::FindSystemBinary ("dmsetup", dmsetupError).empty())
+			throw NotApplicable (SRC_POS);
+
 		// Load device mapper kernel module
 		list <string> execArgs;
 		foreach (const string &dmModule, StringConverter::Split ("dm_mod dm-mod dm"))