mount-util: fix error code

mount-util: fix error code

If multiple service is starting simultaneously with a shared image,
then one of the service may fail to create a mount node:

systemd[695]: Bind-mounting /usr/lib/os-release on /run/systemd/unit-root/run/host/os-release (MS_BIND|MS_REC "")...
systemd[696]: Bind-mounting /usr/lib/os-release on /run/systemd/unit-root/run/host/os-release (MS_BIND|MS_REC "")...
systemd[695]: Failed to mount /usr/lib/os-release (type n/a) on /run/systemd/unit-root/run/host/os-release (MS_BIND|MS_REC ""): No such file or directory
systemd[696]: Failed to mount /usr/lib/os-release (type n/a) on /run/systemd/unit-root/run/host/os-release (MS_BIND|MS_REC ""): No such file or directory
systemd[695]: Bind-mounting /usr/lib/os-release on /run/systemd/unit-root/run/host/os-release (MS_BIND|MS_REC "")...
systemd[696]: Failed to create destination mount point node '/run/systemd/unit-root/run/host/os-release': Operation not permitted
systemd[695]: Successfully mounted /usr/lib/os-release to /run/systemd/unit-root/run/host/os-release

The function apply_one_mount() in src/core/namespace.c gracefully
handles -EEXIST from make_mount_point_inode_from_path(), but it erroneously
returned -EPERM previously. This fixes the issue.

Fixes one of the issues in #24147, especially reported at
https://github.com/systemd/systemd/issues/24147#issuecomment-1236194671.