vfs: fix regression introduced in 1c916c5

we have to consult export file on non pseudo inodes only.
This regression prevent clients to traverse PseudoFS.

Acked-by: Karsten Schwank
Target: master, 0.7
(cherry picked from commit 1c89149d3fdf2ad48b082088314891c5078bed44)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>