call chase_symlinks without the /sysroot prefix (#6411)

In case fstab-generator is called in the initrd, chase_symlinks()
returns with a canonical path "/sysroot/sysroot/<mountpoint>", if the
"/sysroot" prefix is present in the path.

This patch skips the "/sysroot" prefix for the chase_symlinks() call,
because "/sysroot" is already the root directory and chase_symlinks()
prepends the root directory in the canonical path returned.

src/fstab-generator/fstab-generator.c

@@ -526,7 +526,7 @@ static int parse_fstab(bool initrd) {
                         continue;
                 }
 
-                where = initrd ? strappend("/sysroot/", me->mnt_dir) : strdup(me->mnt_dir);
+                where = strdup(me->mnt_dir);
                 if (!where)
                         return log_oom();