pattern: Add support for files in buffer_pattern argument.

pattern: Add support for files in buffer_pattern argument.

It is useful to be able to initialize the buffer contents using an
input file (e.g. when testing certain pathological patterns for
compression). Add support to the buffer_pattern input argument for
reading data from a file via enclosing the filename in ``''``.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-By: Muli Ben-Yehuda <muli@lightbitslabs.com>
Signed-off-by: Stephen Bates <sbates@raithlin.com>
Signed-off-by: Jens Axboe <axboe@fb.com>