Do not lstat common path prefix

When both path have identical prefix i.e. /dev/disk/by-id
skip  2 x lstat() for /dev  /dev/disk /dev/disk/by-id
and directly lstat() only different part of the path.

Reduces amount of lstat calls on system with lots of devices.

WHATS_NEW

@@ -1,5 +1,6 @@
 Version 2.02.89 - 
 ==================================
+  Reduce number of lstat calls when selecting device alias.
   Add function to inialize common structure device members.
   Always zalloc device structure during initialization.
   Fix missing thread list manipulation protection in dmeventd.

lib/device/dev-cache.c

@@ -259,6 +259,13 @@ static int _compare_paths(const char *path0, const char *path1)
 	s0 = &p0[0] + 1;
 	s1 = &p1[0] + 1;
 
+	/*
+	 * If we reach here, both paths are the same length.
+	 * Now skip past identical path components.
+	 */
+	while (*s0 && *s0 == *s1)
+		s0++, s1++;
+
 	/* We prefer symlinks - they exist for a reason!
 	 * So we prefer a shorter path before the first symlink in the name.
 	 * FIXME Configuration option to invert this? */