remount: Fix failure for system-as-root

Since commit 5ad7b3cb ("Try to remount mounted points only"), the
system-as-root cannot be remounted successfully when execute command
"adb remount".  This is because the mount point "/system" cannot be
found and directly bails out with failure.

Add an extra checking for the mount point "/system" for the
system-as-root case, takes it as a found entry; thus the function can
continue to run and fix up to remount root.

Suggested-by: David Anderson <dvander@google.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Change-Id: Ia936c9d97bed951184813a087c70fe591cb33fe0