vfs: remove get_xip_mem

All callers of get_xip_mem() are now gone.  Remove checks for it,
initialisers of it, documentation of it and the only implementation of it.
 Also remove mm/filemap_xip.c as it is now empty.  Also remove
documentation of the long-gone get_xip_page().

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: Boaz Harrosh <boaz@plexistor.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/fadvise.c b/mm/fadvise.c
index fac23ec..4a3907c 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -28,6 +28,7 @@
 SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice)
 {
 	struct fd f = fdget(fd);
+	struct inode *inode;
 	struct address_space *mapping;
 	struct backing_dev_info *bdi;
 	loff_t endbyte;			/* inclusive */
@@ -39,7 +40,8 @@
 	if (!f.file)
 		return -EBADF;
 
-	if (S_ISFIFO(file_inode(f.file)->i_mode)) {
+	inode = file_inode(f.file);
+	if (S_ISFIFO(inode->i_mode)) {
 		ret = -ESPIPE;
 		goto out;
 	}
@@ -50,7 +52,7 @@
 		goto out;
 	}
 
-	if (mapping->a_ops->get_xip_mem) {
+	if (IS_DAX(inode)) {
 		switch (advice) {
 		case POSIX_FADV_NORMAL:
 		case POSIX_FADV_RANDOM: