mount: fix write_offset, #9245 #5372

Job	Run time
lint	9s
asan_ubsan	4m 34s
native_tests (ubuntu-22.04, 3.10, py310-fuse2)	10m 21s
vm_tests (openbsd, 7.7, OpenBSD, false)	20m 31s
native_tests (ubuntu-24.04, 3.14, py314-fuse3)	10m 23s
vm_tests (freebsd, 14.3, FreeBSD, true, borg-freebsd-14-x86_64-gh)	13m 13s
vm_tests (haiku, r1beta5, Haiku, false)	22m 41s
vm_tests (netbsd, 10.1, NetBSD, false)	13m 30s
	1h 35m 22s

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