BUG_ON conversion for fs/reiserfs

This patch converts several if () BUG(); construct to BUG_ON();
which occupies less space, uses unlikely and is safer when
BUG() is disabled. S_ISREG() has no side effects, so the
conversion is safe.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>