Fix gcc warnings from:
(try compilation with "-Wall -Wp,-D_FORTIFY_SOURCE=2")
mount_mntent.c:28: warning: pointer targets in passing argument 1 of 'strlen' differ in signedness
mount_mntent.c:129: warning: pointer targets in passing argument 1 of 'mangle' differ in signedness
mount_mntent.c:130: warning: pointer targets in passing argument 1 of 'mangle' differ in signedness
mount_mntent.c:131: warning: pointer targets in passing argument 1 of 'mangle' differ in signedness
mount_mntent.c:132: warning: pointer targets in passing argument 1 of 'mangle' differ in signedness
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>