libelf: Only use posix_fallocate when using mmap. Ignore unsupported errors.

Don't use posix_fallocate when not using mmap. It is enough to ftruncate
the file to the right size. pwrite will report an error if there is no
disk space left. And on file systems that don't support fallocate it
might duplicate writes in that case. When using posix_fallocate do ignore
most errors. Other libc implementations don't guarantee the call actually
works always and even with glibc there might be an unexpected error from
the fallback code when the file system doesn't support fallocate. That is
pretty nasty since we might get a SIGBUS in that case when writing to the
mmapped memory. But the chance of that happening is very small.  And will
normally never happen with glibc. So only report an error when
posix_fallocate reports ENOSPC.

Signed-off-by: Mark Wielaard <[email protected]>

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