Add a __anon_inode_getfd helper

[Christoph Hellwig <hch@xxxxxx>]

Hi Al,

this series (against your work.epoll branch), adds a new
__anon_inode_getfd helper, which exposes the functionality in
anon_inode_getfd minus installing the file descriptor.  This
allows to clean up a lot of the places that currently open code
the functionality.