staging: exfat: rename buf_cache_t's 'flag' to 'locked'

buf_cache_t.flag is used only for lock.
Change the variable name from 'flag' to 'locked' and remove unused definitions.

Reviewed-by: Takahiro Mori <Mori.Takahiro@ab.MitsubishiElectric.co.jp>
Signed-off-by: Tetsuhiro Kohada <Kohada.Tetsuhiro@dc.MitsubishiElectric.co.jp>
Link: https://lore.kernel.org/r/20200227104043.11503-1-Kohada.Tetsuhiro@dc.MitsubishiElectric.co.jp


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>