xfs: convert kmem_alloc() to kmalloc()

kmem_alloc() is just a thin wrapper around kmalloc() these days.
Convert everything to use kmalloc() so we can get rid of the
wrapper.

Note: the transaction region allocation in xlog_add_to_transaction()
can be a high order allocation. Converting it to use
kmalloc(__GFP_NOFAIL) results in warnings in the page allocation
code being triggered because the mm subsystem does not want us to
use __GFP_NOFAIL with high order allocations like we've been doing
with the kmem_alloc() wrapper for a couple of decades. Hence this
specific case gets converted to xlog_kvmalloc() rather than
kmalloc() to avoid this issue.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>