nilfs2: divide load_segment_summary function
nilfs2: divide load_segment_summary function

load_segment_summary function has two distinct roles: getting summary
header of a log, and verifying consistencies of the log.

This divide it into two corresponding functions, nilfs_read_log_header
and nilfs_validate_log to clarify the meaning.

Signed-off-by: Ryusuke Konishi <f414f2bdd839fdc054376382fa67e9bc2fbf206f@lab.ntt.co.jp>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs