Improve the diagnostics of buffer pool accesses for B-trees,
so that the file names and line numbers of the real calls are shown
instead of the line of the buf_page_get() call in btr_block_get().

btr_page_get(): Replaced with a macro.

btr_block_get_func(): Renamed from btr_block_get(). Add file, line.

btr_block_get(): A macro that passes the __FILE__, __LINE__ to
btr_block_get_func().

dict_truncate_index_tree(): Replace a btr_page_get() call
with btr_block_get(), since we are only latching the page, not accessing it.