target: writev() on single-element vector is pointless

... in other news:
	filp_open() can't return a struct file with NULL dentry
	filp_open() can't return a struct file negative dentry
	filp_close() of something that never had been in any descriptor
tables is pointless - fput() is all you need

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>