uiomove: Use size_t for the length argument

[des]

Summary

In many if not most cases, the caller has a size_t or off_t. Passing it
as an int may result in overflow.

PR: 298159

[kib]

Did you inspected all callers to ensure that they do not pass negative n/len and rely on n < 0 checks?

sys/amd64/amd64/uio_machdep.c

78

since you are changing the lines anyway

sys/powerpc/powerpc/uio_machdep.c

104

I think the change should add asserts that iov_len >= cnt, uio_resid >= cnt. and most importantly len >= cnt.

[olce]

sys/amd64/amd64/uio_machdep.c

78

Please use != 0 in preference to > 0 for unsigned values. (Same for occurrences below.)