[ARM] Fix shared mmap when more than two maps of the same file exist

[ARM] Fix shared mmap when more than two maps of the same file exist

The shared mmap code works fine for the test case, which only checked
for two shared maps of the same file.  However, three shared maps
result in one mapping remaining cached, resulting in stale data being
visible via that mapping.  Fix this.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>