mm: Override mTHP "file_enabled" defaults at kernel cmdline

Add thp_file= cmdline parameter to allow specifying the default
enablement of each supported file-backed THP size. The parameter accepts
the following format and can be provided multiple times to configure
each size:

  thp_file=<size>[KMG]:<value>

See Documentation/admin-guide/mm/transhuge.rst for more details.

Configuring the defaults at boot time is often necessary because its not
always possible to drop active executable pages from the page cache,
especially if they are well used like libc. The command line parameter
allows configuring the values before the first page is installed in the
page cache.

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