vfs: turn generic_drop_inode() into static inline

Once upon a time it used to be much bigger, but these days there's
no point whatsoever keeping it in fs/inode.c, especially since
it's not even needed as initializer for ->drop_inode() - it's the
default and leaving ->drop_inode NULL will do just as well.

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

fs/inode.c

@@ -1368,17 +1368,6 @@ int generic_delete_inode(struct inode *inode)
 }
 EXPORT_SYMBOL(generic_delete_inode);
 
-/*
- * Normal UNIX filesystem behaviour: delete the
- * inode when the usage count drops to zero, and
- * i_nlink is zero.
- */
-int generic_drop_inode(struct inode *inode)
-{
-	return !inode->i_nlink || inode_unhashed(inode);
-}
-EXPORT_SYMBOL_GPL(generic_drop_inode);
-
 /*
  * Called when we're dropping the last reference
  * to an inode.