shm: Remove silly double assignment

Found a silly double assignment of err is do_shmat. Silly, but good to
clean up the useless code.

Signed-off-by: Neil Horman <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

ipc/shm.c

···
 	if (!sfd)
 		goto out_put_dentry;
 
-	err = -ENOMEM;
-
 	file = alloc_file(path.mnt, path.dentry, f_mode, &shm_file_operations);
 	if (!file)
 		goto out_free;