Add write support to SplitFiemap.

This adds few new methods to introduce write support to SplitFiemap:

(1) Write(), which has an internal cursor to stream data into the split
    files. As the end of one file is reached, the next is opened.
(2) Flush(), which calls fsync() on each internal FiemapWriter.
(3) HasPinnedExtents(), which calls the same on each internal
    FiemapWriter.

Included are some tests for edge cases in Write().

Bug: 126230649
Test: fiemap_writer_test gtest
Change-Id: I9fd509215975dbbb20a44b020315d3c1b287d1a0