gfs2: retry interrupted internal reads

The iomap-based read operations done by gfs2 for its system files, such
as rindex, may sometimes be interrupted and return -EINTR.   This
confuses some users of gfs2_internal_read().  Fix that by retrying
interrupted reads.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>