mm: /proc/pid/smaps: Report contpte mappings

arm64 intends to start using its "contpte" bit in pgtables more
frequently, and therefore it would be useful to know how well utilised
it is in order to help diagnose and fix performance issues.

Add "ContPTEMapped" field, which shows how much of the rss is mapped
using contptes. For architectures that do not support contpte mappings
(as determined by pte_cont() not being defined) the field will be
suppressed.

Rollup Example:

aaaac5150000-ffffccf07000 ---p 00000000 00:00 0                 [rollup]
Rss:               11504 kB
...
ContPTEMapped:      6848 kB

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