fs/jfs: replace ternary operator with min_t()

Fix the following coccicheck warning:

fs/jfs/super.c:748: WARNING opportunity for min().
fs/jfs/super.c:788: WARNING opportunity for min().

min_t() macro is defined in include/linux/minmax.h. It avoids
multiple evaluations of the arguments when non-constant and performs
strict type-checking.

Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>