mtd_blkdevs: add mtd_table_mutex lock back to blktrans_{open, release} to avoid race condition

without lock mtd_table_mutex in blktrans_{open, release}, there will
be a race condition when access devices /dev/mtd1 and /dev/mtdblock1
at the same time with a high frequency open and close test:

kernel BUG at drivers/mtd/mtdcore.c:1221!
Internal error: Oops - BUG: 0 1 PREEMPT_RT SMP
CPU: 0 PID: 15349 Comm: mtd-test Not tainted 5.15.52-rt41-yocto-preempt-rt #1
Hardware name: SoCFPGA Stratix 10 SoCDK (DT)
pstate: 60000005put_mtd_device+0x4c/0x84
lr : blktrans_release+0xb0/0x120
Call trace:
__put_mtd_device+0x4c/0x84
blktrans_release+0xb0/0x120
blkdev_put+0xd4/0x210
blkdev_close+0x34/0x50
__fput+0x8c/0x240
____fput+0x1c/0x30
task_work_run+0x98/00t_64_sync_handler+0xa4/0x130
el0t_64_sync+0x1a0/0x1a4

since the original purpose of commit 799ae31c ("mtd_blkdevs:
don't hold del_mtd_blktrans_dev in blktrans_{open, release}") is
to fix a DEADLOCK issue, here convert mutex_lock to mutex_trylock
and return immediately if failed acquire mtd_table_mutex, then
both race condition and DEADLOCK can be avoided.

Fixes: 799ae31c

 ("mtd_blkdevs: don't hold del_mtd_blktrans_dev in blktrans_{open, release}")
Signed-off-by: Liwei Song <liwei.song@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>