Typo ptxmode instead of ptmxmode in mount option parsing

[eriksjolund]

Both ptxmode and ptmxmode are used in the code.

$ git clone -q https://github.com/containers/podman.git
$ cd podman
$ git grep ptxmode main
main:pkg/specgenutil/volumes.go:                case "uid", "gid", "mode", "ptxmode", "newinstance", "max":
$ git grep ptmxmode main
main:pkg/specgen/generate/oci_linux.go:                 Options:     []string{"rprivate", "nosuid", "noexec", "newinstance", "ptmxmode=0666", "mode=0620"},
main:vendor/github.com/containers/buildah/run_linux.go:                 Options:     []string{"rprivate", "nosuid", "noexec", "newinstance", "ptmxmode=0666", "mode=0620"},
main:vendor/github.com/opencontainers/runtime-tools/generate/generate.go:                               Options:     []string{"nosuid", "noexec", "newinstance", "ptmxmode=0666", "mode=0620", "gid=5"},
$

Search for ptxmode in the Linux kernel git repo
https://github.com/search?q=repo%3Atorvalds%2Flinux+ptxmode&type=code
result: ptxmode is not found

Search for ptmxmode in the Linux kernel git repo
https://github.com/search?q=repo%3Atorvalds%2Flinux+ptmxmode&type=code
result: ptmxmode is found

Conclusion: ptxmode is propably a typo here:

podman/pkg/specgenutil/volumes.go

Line 563 in d3cd509

case "uid", "gid", "mode", "ptxmode", "newinstance", "max":

Is it a correct analysis?