mount: add support for symlinks

allow mounting by symlink of exported path.
The symlink have to be exported as well.

Acked-by: Karsten Schwank
Acked-by: Paul Millar
Target: master, 0.8, 0.7
(cherry picked from commit 08e7d674c7bfc21716ff421c5af1165e5b3dda63)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>