vfs: Make __vfs_write() static

__vfs_write() was unexported, and removed from <linux/fs.h>, but
forgotten to be made static.

Fixes: eb031849

 ("fs: unexport __vfs_read/__vfs_write")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>