staging: zram: protect zram_reset_device() call

(cherry pick from commit 644d478793c6594277f8ae76954da4ace7ac6f96)

Commit 9b3bb7abcdf2df0f1b2657e6cbc9d06bc2b3b36f (remove
zram_sysfs file (v2)) accidentally made zram_reset_device()
racy. Protect zram_reset_device() call with zram->lock.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Jerome Marchand <jmarchand@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Bug: 24810447
Change-Id: I8d22088bf6a2f292038e4104b71d04eec3d2c60e