This does the following three things:

 (1) Makes the functions in mm/readahead.c only use struct file* to pass to
     readpage(). address_mapping* and file_ra_state* are used instead to keep
     track of readahead stuff.

 (2) Adds a new function do_generic_mapping_read() that is similar to
     do_generic_file_read(), except that it uses a mapping pointer and a
     readahead state pointer to access a file. The file* is only used to pass
     to readpage().

 (3) Turns do_generic_file_read() into an inline function in linux/fs.h that
     simply wraps do_generic_mapping_read().

This should mean that it is no longer necessary to have a struct file to
access a file in this manner. Just an inode or address space should be
sufficient.

It also means alternate read-ahead structures can be maintained.

The reason I want this is that I'm writing a general cache manager for
filesystems such as AFS, NFSv4, and Lustre. Block devices are made available
to the "cache manager" by means of a filesystem that can be mounted. I'm
storing meta data in an inode in the cache, but to scan this at the moment I
need to gain a "struct file" to use with do_generic_file_read().

This involves either creating a dummy dentry and struct file (which will cause
Al Viro to come looking for me with a shotgun), or to use an extra auxilliary
filesystem mounted with do_kern_mount(), neither of which are particularly
appealing.

This patch is the alternative... it provides a function that I can pass an
address_space to. This also allows me to make use of readahead semantics
without having to reinvent them for myself.