Use Linux fallocate system call to preallocate storage for the target
file in copy_file backends based on sendfile and copy_file_range. These
backends are only used when the file size is known beforehand, and
preallocating storage allows to reduce filesystem fragmentation and
get an early error if there's not enough free space on the target
filesystem.
Preallocation is only done as an optimization/hint. On filesystems
that do not support it we continue the data copying process as before.
This is why we aren't using posix_fallocate, because glibc contains
an emulation path that is used when the filesystem doesn't support
the functionality. We don't want this emulation, as it would effectively
double the amount of written data.
