2nd superblock position from partition size.
* fs/nilfs.c: Support 2nd super block in case 1st one is accidently corrupted or not synced properly.
* conf/common.rmk (grub_probe_SOURCES): Add fs/nilfs2.c. (grub_fstest_SOURCES): Likewise. (pkglib_MODULES): Add nilfs2.mod. (nilfs2_mod_SOURCES): New variable. (nilfs2_mod_CFLAGS): Likewise. (nilfs2_mod_LDFLAGS): Likewise. * conf/i386-pc.rmk (grub_setup_SOURCES): Add fs/nilfs2.c. * conf/sparc64-ieee1275.rmk (grub_setup_SOURCES): Add fs/nilfs2.c. * fs/nilfs2.c: New file.