fs/adfs: map: use find_next_bit_le() rather than open coding it

Use find_next_bit_le() to find the end of a fragment in the map rather
than open-coding this functionality.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>