mm: page_cache_add_speculative(): refactor out some code duplication

From: John Hubbard <jhubbard@nvidia.com>

This combines the common elements of these routines:

    page_cache_get_speculative()
    page_cache_add_speculative()

This was anticipated by the original author, as shown by the comment in
commit ce0ad7f0

 ("powerpc/mm: Lockless get_user_pages_fast() for
64-bit (v3)"):

    "Same as above, but add instead of inc (could just be merged)"

There is no intention to introduce any behavioral change, but there is a
small risk of that, due to slightly differing ways of expressing the
TINY_RCU and related configurations.

This also removes the VM_BUG_ON(in_interrupt()) that was in
page_cache_add_speculative(), but not in page_cache_get_speculative().
This provides slightly less detection of such bugs, but it given that it
was only there on the "add" path anyway, we can likely do without it
just fine.

And it removes the
VM_BUG_ON_PAGE(PageCompound(page) && page != compound_head(page), page);
that page_cache_add_speculative() had.

Link: http://lkml.kernel.org/r/20190206231016.22734-2-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>