block/file-posix: Let post-EOF fallocate serialize

The XFS kernel driver has a bug that may cause data corruption for qcow2
images as of qemu commit c8bb23cb.  We can work around it by
treating post-EOF fallocates as serializing up until infinity (INT64_MAX
in practice).

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20191101152510.11719-4-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 292d06b9)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>