libglusterfs: Account for overflow in gf_string2bytesize

gf_string2bytesize will now return an error when a value is greater than
UINT64_MAX and set errno to EOVERFLOW. This is needed because casting a
double value greater than UINT64_MAX to uint64_t will convert it to 0.

BUG: 1017746
Change-Id: I6f96efc1e3a1c236685593a6fb7f806a87e46019
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/6068
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>