new helper: __alloc_fd()

Essentially, alloc_fd() in a files_struct we own a reference to.
Most of the time wanting to use it is a sign of lousy API
design (such as android/binder).  It's *not* a general-purpose
interface; better that than open-coding its guts, but again,
playing with other process' descriptor table is a sign of bad
design.

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

include/linux/fdtable.h

@@ -125,6 +125,9 @@ void reset_files_struct(struct files_struct *);
 int unshare_files(struct files_struct **);
 struct files_struct *dup_fd(struct files_struct *, int *);
 
+extern int __alloc_fd(struct files_struct *files,
+		      unsigned start, unsigned end, unsigned flags);
+
 extern struct kmem_cache *files_cachep;
 
 #endif /* __LINUX_FDTABLE_H */