fs: jfs: fix possible NULL pointer dereference in dbFree()

fs: jfs: fix possible NULL pointer dereference in dbFree()

In our fault-injection testing, the variable "nblocks" in dbFree() can be
zero when kmalloc_array() fails in dtSearch(). In this case, the variable
 "mp" in dbFree() would be NULL and then it is dereferenced in
"write_metapage(mp)".

The failure log is listed as follows:

[   13.824137] BUG: kernel NULL pointer dereference, address: 0000000000000020
...
[   13.827416] RIP: 0010:dbFree+0x5f7/0x910 [jfs]
[   13.834341] Call Trace:
[   13.834540]  <TASK>
[   13.834713]  txFreeMap+0x7b4/0xb10 [jfs]
[   13.835038]  txUpdateMap+0x311/0x650 [jfs]
[   13.835375]  jfs_lazycommit+0x5f2/0xc70 [jfs]
[   13.835726]  ? sched_dynamic_update+0x1b0/0x1b0
[   13.836092]  kthread+0x3c2/0x4a0
[   13.836355]  ? txLockFree+0x160/0x160 [jfs]
[   13.836763]  ? kthread_unuse_mm+0x160/0x160
[   13.837106]  ret_from_fork+0x1f/0x30
[   13.837402]  </TASK>
...

This patch adds a NULL check of "mp" before "write_metapage(mp)" is called.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Zixuan Fu <r33s3n6@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>