arm64: Fix kernel panic in get_arm64_sve during core dump

The coredump logic calls get_arm64_sve twice: once to get the note size,
and once to get the data. The note size calculation depended on the
volatile `PCB_FP_SVEVALID` flag. If this flag was cleared between the
two calls (e.g., due to a context switch clearing the flag to comply
with the ABI), the second call would expect a smaller buffer size than
the first, triggering a KASSERT panic ("invalid size").

Fix this by ensuring the SVE state is saved to the PCB before we decide
whether to use SVE or VFP.

PR: 292195
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D54532

sys/arm64/arm64/vfp.c

@@ -934,6 +934,9 @@ get_arm64_sve(struct regset *rs, struct thread *td, void *buf,
 
 	pcb = td->td_pcb;
 
+	if (td == curthread && (pcb->pcb_fpflags & PCB_FP_STARTED) != 0)
+		vfp_save_state(td, pcb);
+
 	/* If there is no SVE support in HW then we don't support NT_ARM_SVE */
 	if (pcb->pcb_sve_len == 0)
 		return (false);
@@ -955,9 +958,6 @@ get_arm64_sve(struct regset *rs, struct thread *td, void *buf,
 		KASSERT(*sizep == sizeof(struct svereg_header) + buf_size,
 		    ("%s: invalid size", __func__));
 
-		if (td == curthread && (pcb->pcb_fpflags & PCB_FP_STARTED) != 0)
-			vfp_save_state(td, pcb);
-
 		header = buf;
 		memset(header, 0, sizeof(*header));