zfs: Overallocate nvpair buffers on all CHERI platforms

Otherwise the size != NV_ALIGN(size2) check in i_validate_nvpair() can fail for
nvlists created by a purecap userspace when consumed by a hybrid kernel.

This arises for me with nextboot, which uses zfsbootcfg under the hood to invoke
ZFS_IOC_{GET,SET}_BOOTENV.  For a freshly created pool, the boot environment
type will be VB_RAW, and gets converted to a VB_NVLIST upon the first write.
After that, it is re-read from the pool by the kernel and allocated without the
extra space allocated by the NVPAIR_OVER_ALLOCATE_DECODE option.  When
zfsbootcfg subsequently modifies and sets the boot environment again, the nvlist
contains pad bytes inserted by nvpair_nominal_decode_size(), and the
above-mentioned check in i_validate_nvpair() fails.

module/nvpair/nvpair.c

@@ -55,7 +55,7 @@
 
 #define	skip_whitespace(p)	while ((*(p) == ' ') || (*(p) == '\t')) (p)++
 
-#ifdef __CHERI_PURE_CAPABILITY__
+#if __has_feature(capabilities)
 #define NVPAIR_OVER_ALLOCATE_DECODE
 #endif