access_check: root has full access on directories

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
access_check: add missing early return statements

The returns apparently went missing in bd451c42b846ebf84d427972cdb668ea7e14f360
(thanks to Frank for looking into it!)

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
9P: returns EROFS when export entry is read-only.

Change-Id: I65179e57eb1b73ab7f1dda26721ad5e3f488e394
Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>