mount: fix incorrect string length calculation

Fix bug https://code.google.com/p/android/issues/detail?id=54192
which incorrectly calculated the length of a string.

Fix compiler warning:

system/core/toolbox/mount.c:59:2: warning: initializer-string for array of chars is too long [enabled by default]
system/core/toolbox/mount.c:59:2: warning: (near initialization for 'options[16].str') [enabled by default]

Change-Id: If8663f8311c6348a730fcf731d402b57fee10cb5