x86: add a safe variant of MSR_OP_SCHED* operations

[kib]

Summary

The modifier executes the operation using msr{read,write}_safe()
functions instead of plain msr reads and writes.  Returns EFAULT if any
MSR access caused #GP.

x86_msr_op(9): consistently return the value read from MSR

x86_msr_op_one((9): execute SCHED ops in critical section

x86: provide extended description for x86_msr_op(9)

[kib]

finalize the change

[kib]

Minor clarifications to the description.

I do not think that converting the comment into a man page is warranted.

[kib]

Execute SCHED ops in critical section

[kib]

Fix locking error with sched_bind()