blockgroup_lock.h: remove debris from bgl_lock_ptr() conversion

An obsolete comment and extra parentheses were left over from when the
sb_bgl_lock() macro was replaced with the bgl_lock_ptr() function.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>