libsmb: Introduce type-safe struct cli_smb2_create_flags

libsmb: Introduce type-safe struct cli_smb2_create_flags

This makes it clearer what to pass into the create_flags argument to
cli_smb2_create_fnum(). There was already confusion in
source3/torture/test_smb2.c: It passed in
SMB2_OPLOCK_LEVEL_NONE (which was okay because it #defines to 0), but
it should have been a straight 0, for example
SMB2_OPLOCK_LEVEL_EXCLUSIVE would have been wrong.

This way adding other flags (.nofollow comes to mind) will be much
easier to handle.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>