xfs: devirtualize ->m_dirnameops

Instead of causing a relatively expensive indirect call for each
hashing and comparism of a file name in a directory just use an
inline function and a simple branch on the ASCII CI bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: fix unused variable warning]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>