arm: p2m.c bug-fix: hypervisor hang on __p2m_get_mem_access

arm: p2m.c bug-fix: hypervisor hang on __p2m_get_mem_access

When __p2m_get_mem_access gets called, the p2m lock is already taken
by either get_page_from_gva or p2m_get_mem_access.

Possible code paths:
1)	-> get_page_from_gva
		-> p2m_mem_access_check_and_get_page
			-> __p2m_get_mem_access
2)	-> p2m_get_mem_access
		-> __p2m_get_mem_access

In both cases if __p2m_get_mem_access subsequently gets to
call p2m_lookup (happens if !radix_tree_lookup(...)), a hypervisor
hang will occur, since p2m_lookup also spin-locks on the p2m lock.

This bug-fix simply replaces the p2m_lookup call from __p2m_get_mem_access
with a call to __p2m_lookup.

Following Ian's suggestion, we also add an ASSERT to ensure that
the p2m lock is taken upon __p2m_get_mem_access entry.

Signed-off-by: Corneliu ZUZU <czuzu@bitdefender.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>