jbd2: mark file-local functions as static

Mark functions as static in jbd2/journal.c because they are not used
outside this file.

This eliminates the following warning in jbd2/journal.c:
fs/jbd2/journal.c:125:5: warning: no previous prototype for ‘jbd2_verify_csum_type’ [-Wmissing-prototypes]
fs/jbd2/journal.c:146:5: warning: no previous prototype for ‘jbd2_superblock_csum_verify’ [-Wmissing-prototypes]
fs/jbd2/journal.c:154:6: warning: no previous prototype for ‘jbd2_superblock_csum_set’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>