JFS: Fix compiler warning in jfs_logmgr.c

fs/jfs/jfs_logmgr.c: In function `jfs_flush_journal':
fs/jfs/jfs_logmgr.c:1632: warning: unused variable `mp'

Some debug code in jfs_flush_journal does nothing when CONFIG_JFS_DEBUG
is not defined.  Place the whole code segment within an ifdef to avoid
unnecessary code to be compiled and the warning to be issued.

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>