fs, nfs: convert pnfs_layout_segment.pls_refcount from atomic_t to refcount_t

fs, nfs: convert pnfs_layout_segment.pls_refcount from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <[email protected]>
Signed-off-by: Hans Liljestrand <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: David Windsor <[email protected]>
Signed-off-by: Anna Schumaker <[email protected]>