libmisc/btrfs: find btrfs command

Ubuntu for instance keeps it in /bin, not /sbin.  So look
for it in our usual places.

Signed-off-by: Serge Hallyn <shallyn@cisco.com>

libmisc/btrfs.c

@@ -1,14 +1,36 @@
 #include <linux/btrfs_tree.h>
 #include <linux/magic.h>
 #include <sys/statfs.h>
+#include <stdbool.h>
 
 #include "prototypes.h"
 
+static bool path_exists(const char *p)
+{
+	int ret;
+	struct stat sb;
+
+	return stat(p, &sb) == 0;
+}
+
+static const char *btrfs_cmd(void)
+{
+	const char *btrfs_paths[] = {"/sbin/btrfs",
+		"/bin/btrfs", "/usr/sbin/btrfs", "/usr/bin/btrfs", NULL};
+	const char *p;
+	int i;
+
+	for (i = 0, p = btrfs_paths[i]; p; i++, p = btrfs_paths[i])
+		if (path_exists(p))
+			return p;
+
+	return NULL;
+}
 
 static int run_btrfs_subvolume_cmd(const char *subcmd, const char *arg1, const char *arg2)
 {
 	int status = 0;
-	const char *cmd = "/sbin/btrfs";
+	const char *cmd = btrfs_cmd();
 	const char *argv[] = {
 		strrchr(cmd, '/'),
 		"subvolume",