cryptodisk: Move global variables into grub_cryptomount_args struct

cryptodisk: Move global variables into grub_cryptomount_args struct

Note that cargs.search_uuid does not need to be initialized in various parts
of the cryptomount argument parsing, just once when cargs is declared with
a struct initializer. The previous code used a global variable which would
retain the value across cryptomount invocations.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>