fcntl.h: add F_SEAL_FUTURE_WRITE from linux v5.1

needed for android so it can migrate from its ashmem to memfd.
allows making the memfd readonly for future users while keeping
a writable mmap of it. see

  linux commit ab3948f58ff841e51feb845720624665ef5b7ef3
  mm/memfd: add an F_SEAL_FUTURE_WRITE seal to memfd
1 file changed