vfs: define a generic function to read a file from the kernel

For a while it was looked down upon to directly read files from Linux.
These days there exists a few mechanisms in the kernel that do just
this though to load a file into a local buffer.  There are minor but
important checks differences on each.  This patch set is the first
attempt at resolving some of these differences.

This patch introduces a common function for reading files from the kernel
with the corresponding security post-read hook and function.

Changelog v4+:
- export security_kernel_post_read_file() - Fengguang Wu
v3:
- additional bounds checking - Luis
v2:
- To simplify patch review, re-ordered patches

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Reviewed-by: Luis R. Rodriguez <mcgrof@suse.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -2577,6 +2577,7 @@ static inline void i_readcount_inc(struct inode *inode)
 extern int do_pipe_flags(int *, int);
 
 extern int kernel_read(struct file *, loff_t, char *, unsigned long);
+extern int kernel_read_file(struct file *, void **, loff_t *, loff_t);
 extern ssize_t kernel_write(struct file *, const char *, size_t, loff_t);
 extern ssize_t __kernel_write(struct file *, const char *, size_t, loff_t *);
 extern struct file * open_exec(const char *);