tools/vm/slabinfo: indicates the cause of the EACCES error

tools/vm/slabinfo: indicates the cause of the EACCES error

If you don't run slabinfo with a superuser, return 0 when read_slab_dir()
reads get_obj_and_str("slabs", &t), because fopen() fails (sometimes
EACCES), causing slabcache() to return directly, without any error during
this time, we should tell the user about the EACCES problem instead of
running successfully($?=0) without any error printing.

 For example:
 $ ./slabinfo
 Permission denied, Try using superuser  <== What this submission did
 $ sudo ./slabinfo
 Name            Objects Objsize   Space Slabs/Part/Cpu  O/S O %Fr %Ef Flg
 Acpi-Namespace     5950      48  286.7K         65/0/5   85 0   0  99
 Acpi-Operand      13664      72  999.4K       231/0/13   56 0   0  98
 ...

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>