perf: Split out VM accounting

perf: Split out VM accounting

Similarly to the mlock limit calculation the VM accounting is required for
both the ringbuffer and the AUX buffer allocations.

To prepare for splitting them out into separate functions, move the
accounting into a helper function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/r/20250812104018.660347811@infradead.org