btrfs: refactor main loop in memcpy_extent_buffer()

[BACKGROUND]
Currently memcpy_extent_buffer() does a loop where it would stop at
any page boundary inside [dst_offset, dst_offset + len) or [src_offset,
src_offset + len).

This is mostly allowing us to do copy_pages(), but if we're going to use
folios we will need to handle multi-page (the old behavior) or single
folio (the new optimization).

The current code would be a burden for future changes.

[ENHANCEMENT]
There is a hidden pitfall of the naming memcpy_extent_buffer(), unlike
regular memcpy(), this function can handle overlapping ranges.

So here we extract write_extent_buffer() into a new internal helper,
__write_extent_buffer(), and add a new parameter @use_memmove, to
indicate whether we should use memmove() or regular memcpy().

Now we can go __write_extent_buffer() to handle writing into the dst
range, with proper overlapping detection.

This has a tiny change to the chance of calling memmove().
As the split only happens at the source range page boundaries, the
memcpy/memmove() range would be slightly larger than the old code,
thus slightly increase the chance we call memmove() other than memcopy().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>