Fix incorrect file offset calculation in memory mapping #1427

Job	Run time
Decide which wheel types to build	1s
Build source distribution	11s
manylinux_x86_64 wheels	6m 39s
musllinux_x86_64 wheels	8m 3s
test_attaching_to_old_interpreters (2.7)	2m 20s
test_attaching_to_old_interpreters (3.6)	1m 31s
test_in_alpine	49s
test_wheels (3.7)	1m 28s
test_free_threading (3.13)	8m 57s
test_wheels (3.8)	48s
test_wheels_in_fedora	1m 5s
test_wheels (3.9)	5m 59s
test_wheels_in_arch	1m 3s
test_wheels (3.10)	8m 58s
test_wheels_in_debian	8m 45s
test_wheels (3.11)	1m 46s
test_wheels (3.12)	1m 52s
test_wheels (3.13)	2m 19s
upload_pypi	1s
	1h 2m 35s

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