9P: fixes bug #1125830 and make it possible to mount sub-directory via 9p

This bug makes it possible for 9p to mount a sub-directory of an exported directory.
For example, if "/foo" is exported, you now can mount "/foo/bar".
It fixes bug #1125830 too by returning correct error when trying to mount non-existing
directories or non-accessible directories.

Change-Id: I92594dca93dc8e44dfc2f05f28329014d76df4d5
Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>