losetup: clean up gcc warnings

losetup: clean up gcc warnings

Fix strict gcc warnings that come from using:
  ("-Wall -Wp,-D_FORTIFY_SOURCE=2")

lomount.c:98: warning: pointer targets in initialization differ in signedness
lomount.c:111: warning: format '%04x' expects type 'unsigned int', but argument 4 has type '__kernel_old_dev_t'
lomount.c:300: warning: pointer targets in passing argument 1 of 'xstrncpy' differ in signedness
lomount.c:307: warning: pointer targets in passing argument 1 of '__builtin___snprintf_chk' differ in signedness
lomount.c:339: warning: pointer targets in passing argument 1 of 'xstrncpy' differ in signedness

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>