fatfs: switch write_lock to read_lock in fat_ioctl_get_attributes

fatfs: switch write_lock to read_lock in fat_ioctl_get_attributes

There is no need to hold write_lock in fat_ioctl_get_attributes.
write_lock may make an impact on concurrency of fat_ioctl_get_attributes.

Signed-off-by: Yubo Feng <fengyubo3@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Link: http://lkml.kernel.org/r/1593308053-12702-1-git-send-email-fengyubo3@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>