mips/vdso: Support mremap() for vDSO

mips/vdso: Support mremap() for vDSO

vDSO VMA address is saved in mm_context for the purpose of using
restorer from vDSO page to return to userspace after signal handling.

In Checkpoint Restore in Userspace (CRIU) project we place vDSO VMA
on restore back to the place where it was on the dump.

Make vDSO code track the VMA address by supplying .mremap() fops
the same way it's done for x86 and arm by:
commit 6eb1736a6083 ("x86/vdso: Add mremap hook to vm_special_mapping")
commit a210f642592c ("arm64/vdso: Support mremap() for vDSO").

Signed-off-by: Guoyun Sun <sunguoyun@loongson.cn>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Allison Randal <allison@lohutok.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org