fs/proc/base: use maple tree iterators in place of linked list

fs/proc/base: use maple tree iterators in place of linked list

Link: https://lkml.kernel.org/r/20220504011345.662299-27-Liam.Howlett@oracle.com
Link: https://lkml.kernel.org/r/20220621204632.3370049-43-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>