f2fs: introduce get_available_block_count() for cleanup

There are very similar codes in inc_valid_block_count() and
inc_valid_node_count() which is used for available user block
count calculation.

This patch introduces a new helper get_available_block_count()
to include those common codes, and used it to clean up codes.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>