Fix building on musl

* Add missing linux/stddef.h includes
* Explicitly cast PAGE_SIZE to size_t. PAGE_SIZE is defined without UL
  suffix in musl
* Musl doesn't define PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP, so
  initialize the mutexes with pthread_once.

include/linux/slab.h

@@ -22,7 +22,7 @@ static inline void *kmalloc(size_t size, gfp_t flags)
 
 	p = size
 	    ? aligned_alloc(min(rounddown_pow_of_two(size),
-				PAGE_SIZE), size)
+				(size_t)PAGE_SIZE), size)
 	    : malloc(0);
 	if (p && (flags & __GFP_ZERO))
 		memset(p, 0, size);

libbcachefs/bcachefs.h

@@ -179,6 +179,7 @@
 #undef pr_fmt
 #define pr_fmt(fmt) "bcachefs: %s() " fmt "\n", __func__
 
+#include <linux/stddef.h>
 #include <linux/bug.h>
 #include <linux/bio.h>
 #include <linux/closure.h>

libbcachefs/bcachefs_format.h

@@ -72,6 +72,7 @@
  * inode number, 64 bit offset, 96 bit version field, etc.) for negligible cost.
  */
 
+#include <linux/stddef.h>
 #include <asm/types.h>
 #include <asm/byteorder.h>
 #include <linux/kernel.h>

libbcachefs/siphash.c

@@ -44,6 +44,7 @@
  * https://131002.net/siphash/
  */
 
+#include <linux/stddef.h>
 #include <asm/byteorder.h>
 #include <asm/unaligned.h>
 #include <linux/bitops.h>

linux/preempt.c

@@ -15,7 +15,16 @@
  * correct to instead guarantee mutual exclusion for the critical sections.
  */
 
-static pthread_mutex_t preempt_lock = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;
+static pthread_mutex_t preempt_lock;
+
+__attribute__((constructor))
+static void preempt_init(void) {
+	pthread_mutexattr_t attr;
+	pthread_mutexattr_init(&attr);
+	pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
+	pthread_mutex_init(&preempt_lock, &attr);
+	pthread_mutexattr_destroy(&attr);
+}
 
 void preempt_disable(void)
 {