libsmack: fix smack_have_access() (regression in d7319c71)

Commit d7319c71 introduced an internal function for opening smackfs files,
when there is a long and short label version. The new function always
opens the file write only, but smack_have access() requires O_RDWR.
The internal function is now extended to take argument with file access
mode.

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>