fs: befs: Remove redundant validation from befs_find_brun_direct

The only caller of befs_find_brun_direct is befs_fblock2brun, which
already validates that the block is within the range of direct blocks.
So remove the duplicate validation.

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