Add support for writable MAP_SHARED mmap()ings. Avoid issues with late
writepage()s building transactions by doing the block_write_begin() work in
scoutfs_data_page_mkwrite(). Ensure the page is marked dirty and prepared
for write, then let the VM complete the write when the page is flushed or
invalidated.
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>