x86/mm: Fix function name typo in pmd_read_atomic() comment

x86/mm: Fix function name typo in pmd_read_atomic() comment

The function involved should be pte_offset_map_lock() and we never have
function pmd_offset_map_lock defined.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190925014453.20236-1-richardw.yang@linux.intel.com
[ Minor edits. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>