fat: delete an unnecessary check before brelse()

brelse() tests whether its argument is NULL and then returns immediately.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Link: http://lkml.kernel.org/r/cfff3b81-fb5d-af26-7b5e-724266509045@web.de
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>