cifs: fix oplock request handling in posix codepath

cifs_posix_open takes a "poplock" argument that's intended to be used in
the actual posix open call to set the "Flags" field. It ignores this
value however and declares an "oplock" parameter on the stack that it
passes uninitialized to the CIFSPOSIXOpen function. Not only does this
mean that the oplock request flags are bogus, but the result that's
expected to be in that variable is unchanged.

Fix this, and also clean up the type of the oplock parameter used. Since
it's expected to be __u32, we should use that everywhere and not
implicitly cast it from a signed type.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>