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.

(cherry picked from commit 98eda38a)