debugfs: add support for more elaborate ->d_fsdata

Currently, the user provided fops, "real_fops", are stored directly into
->d_fsdata.

In order to be able to store more per-file state and thus prepare for more
granular file removal protection, wrap the real_fops into a dynamically
allocated container struct, debugfs_fsdata.

A struct debugfs_fsdata gets allocated at file creation and freed from the
newly intoduced ->d_release().

Finally, move the implementation of debugfs_real_fops() out of the public
debugfs header such that struct debugfs_fsdata's declaration can be kept
private.

Change-Id: I3517697b316d5ed1bbc9e5b24b66463479c6fd4d
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 7c8d469877b16d2c1cecf101a0abb7b218db85bc
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>