fuse: add .write_inode

...and flush mtime from this.  This allows us to use the kernel
infrastructure for writing out dirty metadata (mtime at this point, but
ctime in the next patches and also maybe atime).

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

diff --cc fs/fuse/dir.c
Simple merge

diff --cc fs/fuse/file.c
Simple merge

diff --cc fs/fuse/fuse_i.h
Simple merge

diff --cc fs/fuse/inode.c
Simple merge