change the calling conventions for vfs_parse_fs_string()

Absolute majority of callers are passing the 4th argument equal to
strlen() of the 3rd one.

Drop the v_size argument, add vfs_parse_fs_qstr() for the cases that
want independent length.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>