xprtrdma: removed asm-generic headers from verbs.c

asm-generic/barrier.h and asm/bitops.h are already brought into the
file through the header linux/sunrpc/svc_rdma.h and the file can
still be built with their removal. They have been replaced with the
preferred linux/bitops.h and asm/barrier.h to remove the need for the
asm-generic header.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tanzir Hasan <tanzirh@google.com>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>