If we attempted reschedule two times without suceeding, uiomove will yield,

If we attempted reschedule two times without suceeding, uiomove will yield,
giving other processes a chance to run.

A process feeding a huge buffer to {read,write}{,v} on a file that doesn't
need to wait for I/O, could have hogged a lot of cpu in the kernel, blocking
all userland activity.

Based on a similiar fix in FreeBSD.