UBIFS: fix error path in dbg_debugfs_init_fs
UBIFS: fix error path in dbg_debugfs_init_fs

The debug interface is substandard and on error returns either
NULL or an error code packed in the pointer. So using "IS_ERR"
for the pointers returned by debugfs function is incorrect.
Instead, we should use IS_ERR_OR_NULL.

This path is an improved vestion of the original patch from
Phil Carmody.

Reported-by: Phil Carmody <b694b4b08431b3bb23c9b1e9882202a88c5cc143@nokia.com>
Signed-off-by: Artem Bityutskiy <19b5733dcea388885746d36043d3568bba5b4df7@nokia.com>
Acked-by: Phil Carmody <b694b4b08431b3bb23c9b1e9882202a88c5cc143@nokia.com>

KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs