rseq: Implement rseq_grant_slice_extension()

Provide the actual decision function, which decides whether a time slice
extension is granted in the exit to user mode path when NEED_RESCHED is
evaluated.

The decision is made in two stages. First an inline quick check to avoid
going into the actual decision function. This checks whether:

#1 the functionality is enabled

#2 the exit is a return from interrupt to user mode

#3 any TIF bit, which causes extra work is set. That includes TIF_RSEQ,
which means the task was already scheduled out.

The slow path, which implements the actual user space ABI, is invoked
when:

A) #1 is true, #2 is true and #3 is false

It checks whether user space requested a slice extension by setting
the request bit in the rseq slice_ctrl field. If so, it grants the
extension and stores the slice expiry time, so that the actual exit
code can double check whether the slice is already exhausted before
going back.

B) #1 - #3 are true _and_ a slice extension was granted in a previous
loop iteration

In this case the grant is revoked.

In case that the user space access faults or invalid state is detected, the
task is terminated with SIGSEGV.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251215155709.195303303@linutronix.de