x86/sgx: Support VA page allocation without reclaiming

struct sgx_encl should be protected with the mutex
sgx_encl->lock. One exception is sgx_encl->page_cnt that
is incremented (in sgx_encl_grow()) when an enclave page
is added to the enclave. The reason the mutex is not held
is to allow the reclaimer to be called directly if there are
no EPC pages (in support of a new VA page) available at the time.

Incrementing sgx_encl->page_cnt without sgc_encl->lock held
is currently (before SGX2) safe from concurrent updates because
all paths in which sgx_encl_grow() is called occur before
enclave initialization and are protected with an atomic
operation on SGX_ENCL_IOCTL.

SGX2 includes support for dynamically adding pages after
enclave initialization where the protection of SGX_ENCL_IOCTL
is not available.

Make direct reclaim of EPC pages optional when new VA pages
are added to the enclave. Essentially the existing "reclaim"
flag used when regular EPC pages are added to an enclave
becomes available to the caller when used to allocate VA pages
instead of always being "true".

When adding pages without invoking the reclaimer it is possible
to do so with sgx_encl->lock held, gaining its protection against
concurrent updates to sgx_encl->page_cnt after enclave
initialization.

No functional change.

Reported-by: Haitao Huang <haitao.huang@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lkml.kernel.org/r/42c5934c229982ee67982bb97c6ab34bde758620.1652137848.git.reinette.chatre@intel.com