[PATCH] arm: fix undefined reference to generic_fls

This patch defines constant_fls() instead of removed generic_fls().

Signed-off-by: Akinobu Mita <mita@miraclelinux.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>