asm-generic: pgalloc: provide generic p4d_{alloc_one,free}

Four architectures currently implement 5-level pgtables: arm64, riscv, x86
and s390.  The first three have essentially the same implementation for
p4d_alloc_one() and p4d_free(), so we've got an opportunity to reduce
duplication like at the lower levels.

Provide a generic version of p4d_alloc_one() and p4d_free(), and make use
of it on those architectures.

Their implementation is the same as at PUD level, except that p4d_free()
performs a runtime check by calling mm_p4d_folded().  5-level pgtables
depend on a runtime-detected hardware feature on all supported
architectures, so we might as well include this check in the generic
implementation.  No runtime check is required in p4d_alloc_one() as the
top-level p4d_alloc() already does the required check.

Link: https://lkml.kernel.org/r/26d69c74a29183ecc335b9b407040d8e4cd70c6a.1736317725.git.zhengqi.arch@bytedance.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>		[asm-generic]
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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