xfs: rename xfs_has_attr()

xfs_has_attr() is poorly named. It has global scope as it is defined
in a header file, but it has no namespace scope that tells us what
it is checking has attributes. It's not even clear what "has_attr"
means, because what it is actually doing is an attribute fork lookup
to see if the attribute exists.

Upcoming patches use this "xfs_has_<foo>" namespace for global
filesystem features, which conflicts with this function.

Rename xfs_has_attr() to xfs_attr_lookup() and make it a static
function, freeing up the "xfs_has_" namespace for global scope
usage.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>