libdm: Make ExtractBlockDeviceName() return its result

From https://engdoc.corp.google.com/eng/doc/devguide/cpp/styleguide.md

:
"Prefer using return values over output parameters: they improve
readability, and often provide the same or better performance (see the C++
Primer)." Implement this advice for ExtractBlockDeviceName(). This patch
does not change any functionality.

Bug: 194450129
Test: mm libfs_mgr libdm_test
Change-Id: I6363781163eba08e6128507b403200f472f68a59
Signed-off-by: Bart Van Assche <bvanassche@google.com>