libfiemap: Fix fallocate() usage.

This fixes two bugs in how we use fallocate(). First, using ZERO_RANGE
is a mechanism to allocate holes, not blocks. Instead we should be
passing no flags.

The reason this code worked is because of our explicit writes to the
file, which is intended to defeat lazy block allocation. This is not
necessary for F2FS with proper file pinning support, so we now skip this
when possible.

Together, these fixes improve the speed of "adb remount" when backed
by libfiemap.

Bug: 134949511
Test: adb remount when backed by /data
Change-Id: I746053d183e0c83d56979d9c3fa6d96b40b5e478