uapi: add new system call ABI codepoints for ext4 3.18 backport

note: this doesn't guarantee that functionality provided by
FALLOC_FL_COLLAPSE_RANGE, FALLOC_FL_ZERO_RANGE, and FIEMAP_FLAG_CACHE
to necessarily _work_; it only allows ext4 from 3.18 to *compile*.

Fortunately, these are exotic bits of functionality that most people
never use.

Change-Id: I15640225884c49ad66bf54b9b3c23892497f78de
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@google.com>