Filemap: introduce a FilemapSeek class

Filemap: introduce a FilemapSeek class

This patch introduce a so far dummy FilemapSeek class which sill be an
alternative to FilemapFiemap. It also introduces the 'Filemap' function which
automatically selects which class will be used for getting block map. The
FIEMAP method is preferred as it is supposedly faster (at least for large
enough files).

This patch also converts all places where we creant an instance of 'Fiemap'
class to use the 'Filemap' function.

Change-Id: Ic0fcb060b5262e0314582ef3b6dc5f464e4c39a5
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>