take descriptor-related part of close() to file.c

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

include/linux/fdtable.h

@@ -123,6 +123,8 @@ extern int __alloc_fd(struct files_struct *files,
 		      unsigned start, unsigned end, unsigned flags);
 extern void __fd_install(struct files_struct *files,
 		      unsigned int fd, struct file *file);
+extern int __close_fd(struct files_struct *files,
+		      unsigned int fd);
 
 extern struct kmem_cache *files_cachep;