util/misc: support kernels where `MFD_NOEXEC_SEAL` does not enable sealing

[pobrn]

I am trying to squash the kernel's peculiar behaviour of implicitly enabling sealing on MFD_NOEXEC_SEAL. dbus-broker is one of the affected programs, but as noted below, only the test suite would break, normal operations wouldn't. At least as far as I can tell.

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Newer Linux kernels do not unset F_SEAL_SEAL when MFD_NOEXEC_SEAL is supplied to memfd_create(). Currently, misc_memfd() has a workaround for this peculiarity of the kernel, however, this workaround will break.

Avoid the breakage by checking if the kernel added F_SEAL_SEAL, and only try to add it if it is not already present.

Note that this kernel uAPI change does not affect dbus-broker's normal operations since it never attempts to create a memfd with MFD_NOEXEC_SEAL and without MFD_ALLOW_SEALING. Only the test suite would be broken on newer kernels without this change.

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v2: https://lore.kernel.org/linux-mm/[email protected]/ v1: https://lore.kernel.org/linux-mm/[email protected]/