part: Fix return value of get_max_part_size for ungrowable parts
part: Fix return value of get_max_part_size for ungrowable parts

This function checks for free space after the partition so it
returns 0 for partitions without free space after them which
doesn't really make sense. This changes the function behaviour to
return size of the partition instead.

rhinstaller/libblockdev,rhinstaller/libblockdev,rhinstaller/libblockdev