btrfs: fix iomap_begin length for nocow writes

can_nocow_extent can reduce the len passed in, which needs to be
propagated to btrfs_dio_iomap_begin so that iomap does not submit
more data then is mapped.

This problems exists since the btrfs_get_blocks_direct helper was added
in commit c5794e51 ("btrfs: Factor out write portion of
btrfs_get_blocks_direct"), but the ordered_extent splitting added in
commit b73a6fd1 ("btrfs: split partial dio bios before submit")
added a WARN_ON that made a syzkaller test fail.

Reported-by:  <syzbot+ee90502d5c8fd1d0dd93@syzkaller.appspotmail.com>
Fixes: c5794e51 ("btrfs: Factor out write portion of btrfs_get_blocks_direct")
CC: stable@vger.kernel.org # 6.1+
Tested-by:  <syzbot+ee90502d5c8fd1d0dd93@syzkaller.appspotmail.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>