ARM: Improve vmalloc address checking in show_data()

There is an unhandled corner case with the current check
which has recently been hit. The nbytes argument may be
large enough for the range being dumped to cross a
boundary between page-table mappings, and dump addresses
which are vmalloc'd in the neighboring mapping.

Check every address being dumped to see if its vmalloc'd,
not just the address passed to show_data().

Change-Id: I255b093808177321f5202dcacea337ee333bfc63
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>