s390/pkey: Use preallocated memory for retrieve of UV secret metadata

The pkey uv functions may be called in a situation where memory
allocations which trigger IO operations are not allowed. An example:
decryption of the swap partition with protected key (PAES).

The pkey uv code takes care of this by holding one preallocated
struct uv_secret_list to be used with the new UV function
uv_find_secret(). The older function uv_get_secret_metadata()
used before always allocates/frees an ephemeral memory buffer.
The preallocated struct is concurrency protected by a mutex.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424133619.16495-23-freude@linux.ibm.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>