blockdev: Promote several bitmap functions to non-static

blockdev: Promote several bitmap functions to non-static

The next patch will split blockdev.c, which will require accessing
some previously-static functions from more than one .c file.  But part
of promoting a function to public is picking a naming scheme that does
not reek of exposing too many internals (two of the three functions
were named starting with 'do_').  To make future code motion easier,
perform the function rename and non-static promotion into its own
patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200513011648.166876-5-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>