x86/intel_rdt: Propagate error in rdt_mount() properly

gcc complains:
"warning: ‘dentry’ may be used uninitialized in this function"

The error exit path in rdt_mount(), which deals with a failure in
rdtgroup_create_info_dir(), does not set the error code in dentry and
returns the uninitialized dentry value.

Add the missing error propagation.

[tglx: Massaged changelog ]
Fixes: 4e978d06dedb ("x86/intel_rdt: Add "info" files to resctrl file system")
Signed-off-by: Shaohua Li <shli@fb.com>
Cc: fenghua.yu@intel.com
Cc: tony.luck@intel.com
Link: http://lkml.kernel.org/r/a56a556f6768dc12cadbf97f49e000189056f90e.1478207143.git.shli@fb.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>