Currently, __debugfs_create_file allocates one struct debugfs_fsdata
instance for every file created. However, there are potentially many
debugfs file around, most of which are never touched by userspace.

Thus, defer the allocations to the first usage, i.e. to the first
debugfs_file_get().

A dentry's ->d_fsdata starts out to point to the "real", user provided
fops. After a debugfs_fsdata instance has been allocated (and the real
fops pointer has been moved over into its ->real_fops member),
->d_fsdata is changed to point to it from then on. The two cases are
distinguished by setting BIT(0) for the real fops case.

struct debugfs_fsdata's foremost purpose is to track active users and to
make debugfs_remove() block until they are done. Since no debugfs_fsdata
instance means no active users, make debugfs_remove() return immediately
in this case.

Take care of possible races between debugfs_file_get() and
debugfs_remove(): either debugfs_remove() must see a debugfs_fsdata
instance and thus wait for possible active users or debugfs_file_get() must
see a dead dentry and return immediately.

Make a dentry's ->d_release(), i.e. debugfs_release_dentry(), check whether
->d_fsdata is actually a debugfs_fsdata instance before kfree()ing it.

Similarly, make debugfs_real_fops() check whether ->d_fsdata is actually
a debugfs_fsdata instance before returning it, otherwise emit a warning.

The set of possible error codes returned from debugfs_file_get() has grown
from -EIO to -EIO and -ENOMEM. Make open_proxy_open() and full_proxy_open()
pass the -ENOMEM onwards to their callers.
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>