lib: chunkio: file: fix mmap leak due to missing mremap on syncing (#1037)

When a file is synced, the memory map size can change since there is
always extra space available in the region. The current implementation
did not adjusted the memory mapped size leading to keep mapped regions.

This patch make sure to adjust memory map size if required.

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>