mm: /proc/pid/smaps: Report large folio mappings

With the addition of large folios for page cache pages, it is useful to
see which orders of folios are being mapped into a process.
Additionally, with planned future improvements to allocate large folios
for anonymous memory this will become even more useful. Visibility will
help to tune performance.

New fields "AnonContXXX" and "FileContXXX" indicate physically
contiguous runs of memory, binned into power-of-2 sizes starting with
the page size and ending with the pmd size. Therefore the exact set of
keys will vary by platform. It only includes pte-mapped memory and
reports on anonymous and file-backed memory separately.

Rollup Example:

aaaac9960000-ffffddfdd000 ---p 00000000 00:00 0                 [rollup]
Rss:               10852 kB
...
AnonCont4K:         3480 kB
AnonCont8K:            0 kB
AnonCont16K:           0 kB
AnonCont32K:           0 kB
AnonCont64K:           0 kB
AnonCont128K:          0 kB
AnonCont256K:          0 kB
AnonCont512K:          0 kB
AnonCont1M:            0 kB
AnonCont2M:            0 kB
FileCont4K:         3060 kB
FileCont8K:           40 kB
FileCont16K:        3792 kB
FileCont32K:         160 kB
FileCont64K:         320 kB
FileCont128K:          0 kB
FileCont256K:          0 kB
FileCont512K:          0 kB
FileCont1M:            0 kB
FileCont2M:            0 kB

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>