* gnu/build/file-systems.scm (read-superblock,
null-terminated-latin1->string): New variables.
(sub-bytevector): Move to general section.
(ext2-superblock?, read-ext2-superblock): New variables.
(ext2-superblock-uuid, ext2-superblock-volume-name): Use
sub-bytevector and null-terminated-latin1->string.
(%ext2-sblock-magic, %ext2-sblock-creator-os, %ext2-sblock-uuid,
%ext2-sblock-volume-name): Inline constants.
(luks-superblock?, read-luks-header): New variables.
(%luks-header-size, %luks-magic): Inline.
(partition-label-predicate, partition-uuid-predicate,
luks-partition-uuid-predicate): Use new functions.