Pass $user_id argument for "bp_core_get_user_displayname" filter

[r-a-y]

In BP 1.2.3, would help if the bp_core_get_user_displayname filter passed the $user_id variable in bp-core.php.

On line 1190 in bp-core.php, change:

return apply_filters( 'bp_core_get_user_displayname', $fullname );

to:

return apply_filters( 'bp_core_get_user_displayname', $fullname, $user_id );

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It's a simple request that hopefully makes its way into BP 1.2.4.

[apeatling]

(In [3015]) Fixes #2378 props r-a-y