fs/mbcache: replace __builtin_log2() with ilog2()

Fix compiler error with some gcc version(s) that do not
support __builtin_log2() by replacing __builtin_log2() with
ilog2().

Signed-off-by: T. Makphaibulchoke <tmac@hp.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Maciej W. Rozycki <macro@linux-mips.org>