In coda_free_aux_buf() call debugfs_remove only if buffer entry
is valid (and therfore dentry is valid), double protect by
invalidating dentry value.

Fixes erroneous prematurely dealloc of debugfs caused by
incorrect reference count incrementing.
Signed-off-by: NPeter Seiderer <ps.report@gmx.net>
Signed-off-by: NPhilipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: NKamil Debski <k.debski@samsung.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>