mm/gup: update comment explaining why gup_fast() disables IRQs

mm/gup: update comment explaining why gup_fast() disables IRQs

The current comment in gup_fast() talks about "IPIs that come from THPs
splitting", which is outdated and refers to the old THP splitting
implementation that was removed in commit ad0bed24e98b ("thp: drop all
split_huge_page()-related code"), which landed in v4.5.  Before then, THP
splitting involved a pmdp_splitting_flush(), which sent an IPI to
serialize against gup_fast().

Nowadays, we use tlb_remove_table_sync_one() to send IPIs that serialize
against gup_fast(); this is used, for example, in THP *collapsing* to stop
gup_fast() walks of a page table before depositing it.

Link: https://lkml.kernel.org/r/20250528-gup-irq-comment-fix-v1-1-b9d83c345333@google.com
Signed-off-by: Jann Horn <jannh@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>