fs: befs: Remove useless calls to brelse in befs_find_brun_dblindirect

The calls to brelse are useless since dbl_indir_block and indir_block
are NULL.

Signed-off-by: Salah Triki <salah.triki@gmail.com>
Acked-by: Luis de Bethencourt <luisbg@osg.samsung.com>