When we share memory through FF-A and the description of the buffers
exceeds the size of the mapped buffer, the fragmentation API is used.
The fragmentation API allows specifying chunks of descriptors in subsequent
FF-A fragment calls and no upper limit has been established for this.
The entire memory region transferred is identified by a handle which can be
used to reclaim the transferred memory.
To be able to reclaim the memory, the description of the buffers has to fit
in the ffa_desc_buf.
Add a bounds check on the FF-A sharing path to prevent the memory reclaim
from failing.

Also do_ffa_mem_xfer() does not need __always_inline, except for the
BUILD_BUG_ON() aspect, which gets moved to a macro.

[maz: fixed the BUILD_BUG_ON() breakage with LLVM, thanks to Wei-Lin Chang
 for the timely report]

Fixes: 634d90cf ("KVM: arm64: Handle FFA_MEM_LEND calls from the host")
Cc: stable@vger.kernel.org
Reviewed-by: Sebastian Ene <sebastianene@google.com>
Signed-off-by: Snehal Koukuntla <snehalreddy@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240909180154.3267939-1-snehalreddy@google.comSigned-off-by: Marc Zyngier <maz@kernel.org>