xlators/mgmt: don't allow glusterd fork bomb (cache the brick inode size)

Was don't leave zombies if required programs aren't installed

Also, the existing if (strcmp (foo, bar) == 0) antipattern leaves me
underwhelmed -- table driven is better;

I like fully qualified paths to system tools too.

File systems aren't going to change their inode size. Rather than
fork-and-exec a tool repeatedly, hang on to the answer for subsequent
use. Even if there are hundreds of volumes the size of a dict to keep
this in memory is small.

Change-Id: I704a8b1215446488b6e9e051a3e031af21b37adb
BUG: 1081013
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: http://review.gluster.org/8134
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>