io_uring: IORING_OP_F[GS]ETXATTR is fine with REQ_F_FIXED_FILE

Rejection of IOSQE_FIXED_FILE combined with IORING_OP_[GS]ETXATTR
is fine - these do not take a file descriptor, so such combination
makes no sense.  The checks are misplaced, though - as it is, they
triggers on IORING_OP_F[GS]ETXATTR as well, and those do take
a file reference, no matter the origin.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>