. vfs read_only() assumes vnode->v_vmnt is non-NULL, but it can
be NULL sometimes
. e.g. fchmod() on UDS triggered NULL deref; add a check and
add REQ_CHMOD to pfs so unix domain sockets can be fchmod()ded
. add to test56
Change-Id: I83c840f101b647516897cc99fcf472116d762012